You have changed VHOSTS location using Plesk reconfigurator and Horde stopped working.
Horde home folder is located in the same folder as domains home folders. Horde uses sqlite database that is located in "%plesk_vhosts%webmail/horde/db/horde.db".
This connection is configured in Horde configuration file "%plesk_vhosts%webmail/horde/config/conf.php".
After VHOSTS folder is moved the configuration file is not adjusted.
If you have already moved VHOSTS location then you need to adjust horde configuration file by correcting the $conf['sql']['database']
variable setting up a correct value.
This problem has been solved in Plesk version 8.2
. Consider upgrading
Plesk to the latest version