[imp] User not authorized for Mail

[Dr. Lars Hanke]

I did a fresh install of Horde 5.2.13 on nginx 1.10.2 with PHP 7.0.15 
all as shipped with current Debian Testing. Horde runs as a virtual 
server in its own PHP pool with its own user. It is configured to use 
IMP as authentication backend. IMP uses a remote IMAP server (Dovecot) 
for authentication - and as mail store I hope.

Logging in into Horde works nicely with the IMAP credentials. But on the 
user's dashboard I see /User ___ is not authorized for Mail 
(client.fqdn)/. Clicking on the webmail tab asks for another 
authentication, which fails with the very same IMAP credentials.

I followed the FAQ https://wiki.horde.org/FAQ/Admin/Troubleshoot

  * the session counter on /test.php increases
  * I use the default handler, and I see a session file in
    /var/lib/php/sessions
  * The cookie path is '/', which seems correct, and domain is set as
    $_SERVER['SERVER_NAME']. My firefox has two cookies (Horde,
    horde_secret_key) with domain '.server.fqdn'.

I searched for other entries on the web and actually found lots of 
reports. Most of them were related to upgrade issues and bugs in ancient 
versions. Anyhow, after several hours of reading I couldn't find 
anything that resembled my scenario or gave clear information what to check.

Since I'm absolutely new to Horde, I don't even have a clue how it is 
expected to work. But I'm puzzled that if IMP is used for succesful 
authentication, access to IMP is then denied. But I may simply 
misunderstand some core concept.

I chose not to flood the list with config snippets, since I do not know 
where to start. But I'll gladly post any helpful information. I'd 
appreciate any help to do some structured troubleshooting on my install.

Thanks for your help,

  - lars.