I see, it appears it could be a network connection issue as you mentioned.
Would you be able to see if there are any red entries in the Browser Developer Tools:
- on Chrome, press CTRL+SHIFT+I
- choose the "Network" tab and use the Mattermost service. Observe if there are any entries marked in red
It could be that the Webocket connection is being interrupted by the firewall. This happened to a community member before setting up SSL.