the problem may be related to the bounds of dovecot user and IP connections.
You can check the IMAP connections for each user with this command: doveadm who
Verify in /var/log/imap if you find a message like this:
Xxxxxx dovecot: imap-login: Maximum number of connections from user + IP exceeded (mail_max_userip_connections = 12): user = <email@example.com>, method = PLAIN, rip = 127.0.0.1, lip = 127.0.0.1, secured, Session =<zz/8iz1M1AB/AAAB>
In this case, you can solve this by increasing this limit (default = 12) with these two commands:
config setprop dovecot MaxUserConnectionsPerIp 20
It seems that WebTop 5 uses an excessive number of IMAP connections per user, especially when sharing entire accounts.
I'm investigating the real motive