[horde] "Error: User is not authorized for imp"

[Ernie Dunbar]

After yesterday's difficulty in getting Horde set up (I moved the
directory back to its original installation value and changed back what
configuration I had changed), I'm now up against something totally new. 

I'm able to log in to Horde with a regular user as well as the
administrator, authenticate against our IMAP server (I can see the
successful authentication in the IMAP server's logs), but nobody can use
IMP. I just see that panel filled with the message: 

"ERROR

User is not authorized for imp" 

If I try to click on the "Mail" menu at the top, I apparently get logged
out, which seems strange. I've checked the documentation on this error
at http://wiki.horde.org/FAQ/Admin/Troubleshoot#toc5 and none of that
troubleshooting seems to apply. It doesn't work when I turn cookies off
and use URL-based sessions, and besides, I see the cookies on the server
and in the web browser, with ownership for the user 'www-data'. I've
tried different web browsers on different computers to ensure that
there's nothing wrong at the client end. 

Everything else appears to be working fine. Calendar, Address Book,
Tasks and Notes are all fully functional. Most everything works out of
the database, and that's how IMP was originally configurued, but I
changed the session cookies to the filesystem to more easily see the
cookies and ensure that the permissions were set right.