Sorry for late answer, I wasn’t in front of a PC to check my settings…
If your Neth hostname is not the same as your public domain name, you need to set AllowWopiHost to the domain name you use in your browser to connect to Nextcloud.
config setprop loolwsd AllowWopiHost gtbs.ro
signal-event nethserver-collabora-update
If you use port forwarding to your Nextcloud instance, it may help to enable hairpin nat in the firewall settings if you’re testing from internal LAN:
You may check /var/log/messages or /var/lib/nethserver/nextcloud/nextcloud.log for more detailed error messages.