authentication failed (of local folder)

[henry atting]

Used offlineimap for some years know, now it stopped to work.

 OfflineIMAP 6.4.1
Copyright 2002-2011 John Goerzen & contributors.
Licensed under the GNU GPL v2+ (v2 or any later version).

Account sync post:
 *** Processing account post
 Establishing connection to imap.1und1.de:143
 ERROR: While attempting to sync account 'post'
  authentication failed
 *** Finished account 'post' in 0:00
 
ERROR: Exceptions occurred during the run!
 ERROR: While attempting to sync account 'post'
  authentication failed

Erh, what is this? The problem is _not_ the authentication to the remote
host but to the local folder? What authentication? Again: _Not_ a
certification problem, not a problem with the remote host but with the
acces to the _local_ maildir!