With Plesk 7.5.x, a PHP upgrade to version 5 will cause some issues with Horde. These issues stem from problems with the pear scripts that Horde depends on.
To fix it, run these commands:
# pear upgrade DB # cp -a /usr/share/pear/DB.php /usr/share/pear/DB/ /usr/share/psa-horde/pear/
Credit for this fix goes to Mike J.