Horde webmail installed on a Plesk server returns error: Could not instantiate PDO. PDOException: SQLSTATE[HY000] [1045] Access denied for user ‘horde’@’localhost’ (using password: YES)

Symptoms

When trying to access Horde webmail installed on a Plesk server or going to Horde > Preferences, the following error is displayed:

A fatal error has occurred
Could not instantiate PDO with DSN "mysql:port=3306;protocol=tcp;host=127.0.0.1;dbname=horde". 
PDOException: SQLSTATE[HY000] [1045] Access denied for user 'horde'@'localhost' (using password: YES) 
Details have been logged for the administrator.

Cause

The database user horde cannot access the Horde configuration database with the password saved in the Plesk database

Resolution

1. Connect to the server via RDP/SSH

2. Get the password for the horde database user in the file:

   • For Windows:
     %plesk_dir%Webmailhordeconf.horde.shadow

   • For Linux:
     /etc/psa-webmail/horde/.horde.shadow

3. Access Plesk database:

   # plesk db

4. Run the following queries, replacing <PASSWORD> with the password obtained in Step #2:

   MYSQL_LIN: UPDATE mysql.user SET password = PASSWORD(“<PASSWORD>”) WHERE user = ‘horde’;
   FLUSH PRIVILEGES;