I'm using XN with a GMail account. And while I was manually logged into GMail under a different account (one not configured in XN), GMail suddenly told me that I was being logged out automatically because I'm logging into another account somewhere.
Sorry I didn't copy down the exact error message. I was also unsuccessful trying to reproduce the problem. But I do believe that it was a case of XN's periodic check in the background somehow conflicting with my GMail session in the foreground.
Now I *was* able to reproduce
Now I *was* able to reproduce the problem -- every time. I just was able to get that message -- I got different error messages, telling me something is wrong with my connection. Or I was getting any message at all -- the GMail session simply switched from the account I logged into manually to the account configured in XN all by itself.
Similar situation
I had a similar situation with Facebook accounts... and the given solution is to log in to your account using X-Notifier (click the account in X-Notifier's drop-down list to open it in a new tab) rather than directly/manually logging in. This may not be appropriate if you haven't defined the account in X-Notifier... but has worked well for me with FB.