Plesk Migrator error but migration was successful: Unable to connect: Adaptive Server is unavailable or does not exist (localhost\MSSQLSERVER2014)

Symptoms

• A migration was performed successfully, with databases and databases users being migrated, but an error in Plesk Migrator showed up:

Failed to deploy databases of subscription 'example.com'
Migration tools tried to perform operation in 3 attempts: Failed to execute SQL query: DB-Lib error message 20009, severity 9:
Unable to connect: Adaptive Server is unavailable or does not exist (localhost\MSSQLSERVER2014)

Query: SELECT [name] FROM master.dbo.sysdatabases
Server: .\MSSQLSERVER2014
User: sa
Tried to connect from: the local server

Check that:
1) MSSQL server is running.
2) SQL server browser is running.
3) You can connect to the SQL server with MSSQL Management Studio
(from the server from which migrator tried to connect to the server).
4) Database (if specified) exists on the server.
5) Database user (if specified) can login to the server.
6) There are no firewall rules that could break connection.
7) SQL query is valid.

Cause

MSSQL authentication issues. 

Resolution

The cause for this could've been simple: Server authentication could've been set to either allow SQL Server or Windows Authentication. When one failed, the error was thrown, but then it tried another connection method. Therefore, it was able to connect with one of the authentication methods, and it all migrated.

Therefore, if no data is missing, the warning can be safely ignored, and no action is required.