SQL Server error 0x5 (15404) - could not obtain information about Windows NT group/user

Question

I am using SQL Server 2019 (15.0.4298.1). I am sysadmin on SQL Server and a member of the admin group on the server box.

This is a Dev server, which has 5 main user databases, which I set up, using AD groups for permissions. I am a member of the Admin AD group, which is a member of db_owner. The dbo user uses my credentials (presumably because I created the databases using my credentials). As such, I am not able to add myself back into the database as a user.

On Friday last week, I set up Redgate SQL Source Control and linked it to Bitbucket via Sourcetree. All was fine until I tried to run the Redgate lock object script. Since this, I have been receiving the 0x5 error whenever I try to run any scripts (even SP create or alter).

Most of the advice out there suggests to change the database owner to the sa account. However, I am unable to do this, as I get the error:

Could not obtain information about Windows NT group/user '<myusername>’, error code 0x5. (.Net SqlClient Data Provider)

Server Name: [myservername]
Error Number: 15404
Severity: 16
State: 19
Procedure: RG_SQLSourceControl_DDLTrigger

I would appreciate any advice on how to fix this without having to run a full server restore back to before the source control amendment.

Thank you.

I have tried to change the database owner, as recommended in various posts. This caused the error to be returned.

I have checked the user details in the domain, and they are present and spelled correctly.

I have checked the user permissions in SQL Server and on the server machine - admin.

I have tried to restore the databases, but was unable to complete the restore process, as the process includes a step to create the database users (T-SQL script), which failed to run due to the same error.