[horde] login problems

[Valkai Elod]

I started migrating a pretty small install from horde2 to horde3. I've
got the following problems:

- Horde authenticates, but throws me back to the login page,
displaying no errors.
/var/log/horde3.log says:
Nov 04 13:26:51 HORDE [notice] [horde] Login success for elod
[193.226.111.3] to Horde [on line 76 of "/usr/share/horde3/login.php"]

/var/log/syslog:
Nov  4 13:35:55 nova imaplogin: Connection, ip=[::ffff:127.0.0.1]
Nov  4 13:35:55 nova imaplogin: LOGIN: DEBUG: ip=[::ffff:127.0.0.1],
command=STARTTLS
Nov  4 13:35:55 nova imaplogin: LOGIN: DEBUG: ip=[::ffff:127.0.0.1],
command=CAPABILITY
Nov  4 13:35:55 nova imaplogin: LOGIN: DEBUG: ip=[::ffff:127.0.0.1],
command=LOGIN
Nov  4 13:35:55 nova imaplogin: LOGIN: DEBUG: ip=[::ffff:127.0.0.1],
username=elod
Nov  4 13:35:55 nova imaplogin: LOGIN: DEBUG: ip=[::ffff:127.0.0.1],
password=test
Nov  4 13:35:55 nova imaplogin: authdaemon: starting client module
Nov  4 13:35:55 nova imaplogin: authdaemon: ACCEPT, username elod
Nov  4 13:35:55 nova imaplogin: LOGIN, user=elod,
ip=[::ffff:127.0.0.1], protocol=IMAP
Nov  4 13:35:55 nova imaplogin: LOGOUT, user=elod,
ip=[::ffff:127.0.0.1], headers=0, body=0, time=0, starttls=1

Mail server is courier imap-ssl, distro is a recently upgraded Debian
GNU/Linux running sarge.

/horde/test.php and horde/imp/test.php show no problems other than the
memlimit (which is at 8MB).

Both horde and imp were configured via web, database backend is
postgresql without authentication. PEAR_LOG_DEBUG level shows "Login
success" and some sql queries for session handling, nothing else.