I run 6.5, and all was fine until a few days ago. Suddenly users start to complain that they get logged out a few seconds/minutes after they logged in. Same happens to me (Admin), quite erratically. Sometimes I log in, click around, all is fine. Then suddenly I get logged out.

What can be the reason/cause for this?

too big cookies, know problem and 90% fixed with 6.5.1beta4 (public)