How to resolve error 15517 when DBO is set to SA?
Try this to verify the owner of the database:

SELECT name AS DB, SUSER_SNAME(owner_sid) AS owner FROM sys.databases


If the owner is not sa, you can do ALTER AUTHORIZATION or use the deprecated command sp_changedbowner:

USE [*db*]
GO
EXEC sp_changedbowner 'sa'
GO


Even though it is deprecated, this procedure is still available in later versions and I have had more luck with that command for avoiding putting the database into single user mode.

Here is the sp_changedbowner doco