Unable to remove database server: A database server should not be deleted while databases or database users are hosted there

Applicable to:

When trying to delete database server at Tools & Settings > Database servers, the error below appears:

PLESK_ERROR: A database server should not be deleted while databases or database users are hosted there
In Tools & Settings > Database servers it is seen that there are no databases assigned to the database:

Plesk database inconsistency.

Fixing in Plesk interface:

Log into Plesk;
Go to Extensions > My Extensions > Repair Kit (if there is no such extension - install it using this guide: How to install Plesk extension?);
In Repair Kit find Plesk Database and press Repair:

Fixing over SSH in bulk:

Fixing over SSH manually:

Connect to the server via SSH;
Create Plesk database backup;
Log into psa database:

# plesk db
Find out ID of the database server in question:

MYSQL_LIN: > select * from DatabaseServers;
Obtain this ID and find out remaining database users and databases:

Databases:

MYSQL_LIN: > SELECT id, name FROM data_bases WHERE db_server_id=3;
+----+--------------+
| id | name |
+----+--------------+
| 23 | database_1 |
+----+--------------+
1 row in set (0.00 sec)

Database users:

MYSQL_LIN: > SELECT id, login FROM db_users WHERE db_server_id=3;
+----+--------------+
| id | login |
+----+--------------+
| 25 | john_doe |
+----+--------------+
1 row in set (0.00 sec)

Note: use the obtained on the 4th step ID instead of 3 in this example.
Delete found records:

Database:

MYSQL_LIN: > DELETE FROM data_bases WHERE id=23;

Users:

MYSQL_LIN: > DELETE FROM db_users WHERE id=25;

Note: use the ID's from the previous step accordingly instead of the 23 and 25.