Hello, I have been having this problems for weeks and still cant fix it. This only happens to certain chat meetings and it is very annoying. It keeps saying I am not a member of chat and unable to access to conversation. Also, some of my teammates said that my name is displayed as Unknown User even though I can see my own name. It seems like I joined the meeting but not the chat??
This is not normal behavior, obviously - it would be best if you asked the question in the support forum at answers.microsoft.com so they can help you troubleshoot and potentially look at your logs to try and help you figure out what is happening with your specific scenario.