Unable to log into Horde webmail from particular email account: Horde_Prefs_Scope::serialize() must return a string or NULL

Connect to the server via SSH or RDP.

Backup horde database:

For Linux:

# plesk db dump horde > horde.$(date +%F_%R).sql

For Windows:

plesk db dump horde > C:horde_dump.sql

Delete all entries from horde_prefs table that are related to the affected email account:

Warning: This will delete configurations that were set in Horde UI for this user and they should be recreated anew.

Note: Replace [email protected] with the affected email account name.

# plesk db “DELETE FROM horde.horde_prefs WHERE pref_uid = ‘[email protected]’;”