[imp] User authorization failing for imp (Horde login is ok)

[Kai Harrekilde-Petersen]

Hi,

I am installing Horde on an Ubuntu 14.04.1 LTS machine, using their 
php-horde-webmail package.

Versions:
PHP: 5.5.9-1ubuntu4.5 (test.php says that "this version of PHP has not 
been fully tested with this version of Horde")
Horde Group Webmail Edition 5.1.3
Horde version 5.1.5
Imp version 6.1.6

I am able to login as a user, but the portal services page 
(/horde/services/portal) says "User is not authorized for imp".
I've traced this back to Registry.php, line 1551, when testing for 
$this->isAuthenticated(array('app' => $app))
The isAuthenticated() function throws an exception when doing
         try {
             return $GLOBALS['injector']
->getInstance('Horde_Core_Factory_Auth')
                 ->create($app)
                 ->transparent();
         } catch (Horde_Exception $e) {
             Horde::log($e);
             return false;
         }

I have looked at the Horde wiki' FAQ Admin, and confirmed that sessions 
work, changed $conf['session']['use_only_cookies'], checked the cookie 
path, deleted my cookies - all to no avail.I've googled, but only found 
a message from about a year ago reporting the same, but no solution.
If I navigate to horde/imp, I get logged out and cannot log back in again.

I'm using IMAP for authentication (test.php says OK, I get Login success 
message in my syslog), MySQL for prefs.I used horde-db-migrate to setup 
the tables.

I'm at wit's end, so I'd appreciate any help here.

Thanks,

Kai Harrekilde-Petersen