SymptomsWebmail login page returns the following error instead of showing login screen:
Fatal error: require_once() [function.require]: Failed opening required 'Horde/String.php' (include_path='/usr/share/psa-horde:/usr/share/psa-horde/pear:.') in /usr/share/psa-horde/lib/Horde/Browser.php on line 4
CauseAbility to modify php_admin_value from the PHP scripts has been removed in PHP 5.2 and Webmail cannot add it's include path.
ResolutionThis problem has been corrected since Plesk 8.3.
As workaround in older Plesk versions Webmail VirtualHost can be copied from /etc/httpd/conf.d/zz010_psa_httpd.conf to to /etc/httpd/conf.d/webmail.conf, and php_admin_value include_path value should be changed to the following in the new file:
php_admin_value include_path "/usr/share/psa-horde/lib:/usr/share/psa-horde:/usr/share/psa-horde/pear:."
Restart Apache after that.