Symptoms
When registering a new Microsoft SQL Server in Plesk at Tools & Settings > Database Servers > Add Database Server, the operation fails with:
Failed to connect to the database server: SQLSTATE[28000]: [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]Login failed for user 'administrator'.
Cause
The MS SQL user which was used to register a SQL Server instance does not exist: there is no Administrator user in SQL Management Studio under the section Security > Logins.
Resolution
Add the MS SQL database server using the sa (System Administrator) user.
Note: If the password of the sa user is unknown, reset it.