Database cannot be opened. It is in the middle of a restore. (Error 927)

This post describes how to fix the error:

Database cannot be opened. It is in the middle of a restore. (Microsoft SQL Server, Error: 927)

Cause

This error can occur when you try to configure mirroring using the Configure Database Mirroring Security Wizard.

The endpoints are created successfully:

However, when you try to Start Mirroring:

The error occurs.

Resolution

To resolve the error, I had to start mirroring in SQL directly.

I set the partner to the Principal network address on the mirror SQL instance:

ALTER DATABASE AdventureWorksLT2019 SET PARTNER='TCP://SQL2019-2K19-1.test2019.net:5022'

I then set the partner to the Mirror network address on the principal SQL instance:

ALTER DATABASE AdventureWorksLT2019 SET PARTNER='TCP://SQL2019-2K19-1.test2019.net:5023'

And set the witness to the Witness network address on the principal SQL instance:

ALTER DATABASE AdventureWorksLT2019 SET WITNESS='TCP://SQL2019-2K19-1.test2019.net:5024'

The database should now be in a mirrored / synchronized state:

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