Are you using the integrated authentication? I had a similar problem and just switched to username/pwd for both TR-proper and the forum. See this thread:
Real first world problem here… If I am reading the forum in Firefox and click on the link to get to my calendar. It goes to the login page. If I login there it dumps me out of the forum. I have selected logging with TR on the forum login page… tried the other options too to see if can stay logged in.
What dweebisness am I committing here?
As i said real first world problem there… not really llosing sleep over it but if the fix is trivial I might as well do it.