Exception error in RH10 when I modify and save files

I'm running RH10 integrated with TFS for source control. I can check out projects and files just fine, but when I modify a file in RH and save, I get the following:

This happens every time I save a file and I believe this same issue is causing problems with what's being checked back into TFS. Other users see versions of the file that are different from the one I check in. That is to say, the changes I make prior to check in may or  may not appear to users who check out the file after me.