[horde] One particular user cannot login to Imp

[Otto Stolz]

Hello,

one of our users keeps getting the message "Your Webmail session has 
expired. Please login again.", immediately on every login attempt.
- No problem to access that very IMAP account via other e-mail clients,
   e. g. Mozilla Thunderbird.
- No problem for other users logging in to Webmail.
- I have stopped and restarted Apache, to no avail.

What can I do to re-establish Webmail access for that user?

Thank you in advance,
and best wishes,
    Otto Stolz