SQL SERVER 2016, Restore database get 15517
Can you check what login was mapped to the dbo user in the original SQL Server instance (where the database came from)?
My guess is that the dbo user points to a login that doesn't exists, and that is wnat is setting of this situation.
If that dbo (again, check on the original server) points to a Windows login, then just create that login. If it points to a SQL Server login, then create it using the same SID ad on the originating server. You can for instance use sp_help_revlogin on the originating server to script out the CREATE LOGIN command.
Where did you change the owner by the way? On the originating server? If so, then the login for the dbo user should definitely exist!
Perhaps it is because you who do the restore isn't a sysadmin?