Hi Stefan,
Thank you for the email. I apologize for the delay in getting back to you.
This is an odd issue. The screen shot you provided doesn't have any areas that look to have failed. What I was hoping to see was a request being made that had a parameter "serverInquiry" in it and what the response was. That is the one that looks to see if there are any subscribers that need to be removed b/c of inactivity or sessions, etc. I am assuming the screen shot you included didn't get the "not subscribed" error? If it didn't, can you send a screen shot of a browser that does get the "not subscribed" error.
This is the call we are interested in:
http://127.0.0.1:51234/ilias.php?baseClass=ilObjChatroomGUI&serverInquiry=true
At this point, I would guess the problem is with the chat server (configuration) itself and not a permission issue via the RBAC part since you can't replicate the issue every time (RBAC seems pretty solid). I had the exact same thing happening with not subscribed, but mine was related to the SSL cert and the "man in the middle" whole with secure and insecure content. The URL above was coming back with the response that there were no subscribers and the URL would fail to resolve because of http versus https.
On a side note, we are trying out CometChat the week of December 21st - 23rd. We reached out to them, and they told us that if we like the plugin, they will modify the plugin so that the Comet Chat system can be added as a "Repository Item". I'll keep you posted as well on the status of that system (we got a free copy from Comet Chat to use for development/qa purposes).
To recap, having a screen shot of a failed response in the developer tool bar would help point us in a direction to center in on, whether it is a not resolving URL or a failure to keep active sessions alive in the chat system. I really don't think this is a server firewall issue since you can't replicate this error across different browsers.
Regards,
Adam