Ezt már nézted?
After all this the problem is solved. I had domain\user selected under forms based auth. and I am trying to log in with the username only. Once I used both domain\user format it logged in just fine. I have tested with the other users successfully as well.
A bejelentkezésnél domain\felhasználónév formában bejelentkezést próbáltad?
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trey @ gépház