We've had weird behavior with the web interface going back even to our old versions we had installed.
One issue is the sidebar randomly collapsing and having to fight to keep it to stay open between page refreshes. Another was timeout during impersonation without warning. We've never been logged out randomly from the primary logged in user.
Sometime last week, in several browsers we test with, we deleted the items that the webmail stores in the browser's "local storage" and reset the cookies the site had stored. Since then, the behavior has seemed to stabilize against the sidebar disappearing at least. I don't know if there was old stale data in there from past versions that never got cleaned up - but we'll see with more testing.
MailEnable survivor / convert --