What happens if you log in as the user whose IMAP session you tried to
open, and do:
$ date && touch test && stat test
Dates were identical. The output gave me the clue to the problem,
though. There were two users with same UID on the system. The output of
stat revealed that. When I added a new user with an unique UID login
succeeded.
Perhaps there should be some additional sanity check so that Courier
would not blame clock skew in similar cases? UID mismatch must have
affected to creation of a test file. The other user with same UID did
not have a Maildir. When I tried to login directly with that username,
Courier said "BYE [ALERT] Fatal error: Maildir: No such file or directory".
Thank you for your help to solve the problem.