I saw a ticket the other day where a customer received this error from Horde when trying to expand items on the left pane of the interface:
Fatal error: Cannot use string offset as an array in /www/horde/lib/Horde/Block/Layout/Manager.php on line 389
It turns out that Plesk 8.1.1 bundles Horde 3.1.3 which has an occasional bug within the interface. Upgrading to Plesk 8.2.0 corrects the issue as Horde 3.1.4 is installed with the upgrade.
See Horde’s bug page for more information.