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I'm glad I found this thread when I searched for login problems. This morning on my site, suddenly, nobody could log in. Like a few others here, trying to log in would give the Thanks for Loggin In message, but then you'd be taken back to the login page. What I found, after being alerted to the possibility of a problem in the _session table, was that the latter was corrupted. Doing a Repair Database fixed it, I'm glad to say.
I think it would be useful to have an FAQ on this subject that points out that the _session table needs to be specifically checked.
Barry Cooper
Barry Cooper
Sweet Home, Oregon