[horde] Login problem for Horde Groupware but Fine for Horde/Imp: "user is not authorized for imp"

[balia]

I am running horde 5.2.20 on Debian 8.

First when I login into https://mydomain/horde/imp/, I can login fine  
and I get no error.
I can access the mailboxes without problem.
# openssl s_client -connect mydomain:993 -quiet
There is also no connection problem and I can see all the folders.

But when I login into https://mydomain/horde aka  
https://mydomain/horde/"login.php I get:
  "Error User is not authorized for imp"
There is nothing in the /var/logs that explains the error; neither in  
dovecot.log, horde.log or any other log.
I have verbosity set up at the debug level for both horde and dovecot.
Searching the var/www/horde folder for "User is not authorized for  
imp" or "not authorized for imp" produces no result.
I cannot pinpoint which file is generating the error.

Any help appreciated.
Thank you.


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