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: [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]Login failed for user 'administrator'.
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.
Add the MS SQL database server using the
sa (System Administrator) user.
Note: If the password of the
sa user is unknown, reset it.