Ever get this in your /var/log/httpd/error_log?

PHP Fatal error: Can't load Horde/SessionObjects.php, open_basedir restriction. in /usr/share/psa-horde/lib/Horde/IMAP/Tree.php on line 311

You might also get a 500 error. Its because of eaccelerator, and I think the way its handling paths. The fix is pretty simple, you just need to exclude Horde from its settings by adding the following to /etc/php.d/eaccelerator.ini

eaccelerator.filter = "!*Horde*"

Then restart apache, and test it by logging back in again. Problem should go away