I have this error too. Can’t find the problem. I did untick the box with regards to TLS-validation.
The log file /var/log/messages contains the following.
[Thu Jan 1 01:00:00 1970]May 27 00:57:57 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:58:03 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:58:03 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:58:03 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:58:03 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:58:03 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:58:03 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:58:03 server1 cockpit-ws: New connection to session from 172.21.99.33
[Thu Jan 1 01:00:00 1970]May 27 00:58:39 server1 systemd-logind: New session 2 of user root.
[Thu Jan 1 01:00:00 1970]May 27 00:58:39 server1 systemd: Started Session 2 of user root.
[Thu Jan 1 01:00:00 1970]May 27 00:58:53 server1 systemd: getty@tty1.service has no holdoff time, scheduling restart.
[Thu Jan 1 01:00:00 1970]May 27 00:58:53 server1 systemd: Stopped Getty on tty1.
[Thu Jan 1 01:00:00 1970]May 27 00:58:53 server1 systemd: Started Getty on tty1.
[Thu Jan 1 01:00:00 1970]May 27 00:58:53 server1 systemd-logind: Removed session 2.
[Thu Jan 1 01:00:00 1970]May 27 00:59:27 server1 cockpit-ws: WebSocket from 172.21.99.33 for session closed
[Thu Jan 1 01:00:00 1970]May 27 00:59:27 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:27 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:27 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:30 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:30 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:30 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:30 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:30 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:30 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:31 server1 cockpit-ws: couldn't read from connection: Peer sent fatal TLS alert: Unknown certificate
[Thu Jan 1 01:00:00 1970]May 27 00:59:31 server1 cockpit-ws: New connection to session from 172.21.99.33
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: Traceback (most recent call last):
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 1349, in do_open
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: encode_chunked=req.has_header('Transfer-encoding'))
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 1254, in request
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: self._send_request(method, url, body, headers, encode_chunked)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 1300, in _send_request
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: self.endheaders(body, encode_chunked=encode_chunked)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 1249, in endheaders
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: self._send_output(message_body, encode_chunked=encode_chunked)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 1036, in _send_output
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: self.send(msg)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 974, in send
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: self.connect()
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 946, in connect
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: (self.host,self.port), self.timeout, self.source_address)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/socket.py", line 724, in create_connection
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: raise err
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/socket.py", line 713, in create_connection
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: sock.connect(sa)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: socket.timeout: timed out
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: During handling of the above exception, another exception occurred:
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: Traceback (most recent call last):
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/sbin/ns8-action", line 104, in <module>
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: resp = request.urlopen(req, timeout = 20)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 223, in urlopen
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: return opener.open(url, data, timeout)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 526, in open
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: response = self._open(req, data)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 544, in _open
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: '_open', req)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 504, in _call_chain
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: result = func(*args)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 1377, in http_open
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: return self.do_open(http.client.HTTPConnection, req)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 1351, in do_open
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: raise URLError(err)
[Thu Jan 1 01:00:00 1970]May 27 00:59:51 server1 cockpit-bridge: urllib.error.URLError: <urlopen error timed out>
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: Traceback (most recent call last):
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 1349, in do_open
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: encode_chunked=req.has_header('Transfer-encoding'))
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 1254, in request
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: self._send_request(method, url, body, headers, encode_chunked)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 1300, in _send_request
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: self.endheaders(body, encode_chunked=encode_chunked)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 1249, in endheaders
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: self._send_output(message_body, encode_chunked=encode_chunked)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 1036, in _send_output
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: self.send(msg)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 974, in send
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: self.connect()
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/http/client.py", line 946, in connect
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: (self.host,self.port), self.timeout, self.source_address)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/socket.py", line 724, in create_connection
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: raise err
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/socket.py", line 713, in create_connection
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: sock.connect(sa)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: socket.timeout: timed out
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: During handling of the above exception, another exception occurred:
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: Traceback (most recent call last):
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/sbin/ns8-action", line 104, in <module>
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: resp = request.urlopen(req, timeout = 20)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 223, in urlopen
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: return opener.open(url, data, timeout)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 526, in open
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: response = self._open(req, data)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 544, in _open
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: '_open', req)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 504, in _call_chain
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: result = func(*args)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 1377, in http_open
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: return self.do_open(http.client.HTTPConnection, req)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: File "/usr/lib64/python3.6/urllib/request.py", line 1351, in do_open
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: raise URLError(err)
[Thu Jan 1 01:00:00 1970]May 27 01:00:01 server1 cockpit-bridge: urllib.error.URLError: <urlopen error timed out>
It’s a clean install on Debian 11.