[TYPO3-english] BE Login not possible

[Andreas Cord-Landwehr]

Hello, I've a weird problem. There is no login possible in backend since
last night on my typo3 4.2.8 installation. When I try to login at BE, I
get (password is correct):

Your login attempt did not succeed. Make sure to spell your username and
password correctly, including upper/lowercase characters.

Things I already did:
  * cookies cleared
  * temp-files in typo3-conf deleted
  * be_sessions table cleared
  * encryption key cleared
  * checked that lockIP is 0

I would be very happy about suggestions, what I can do further or where
to get any debug information.

Greetings,
   Andreas