Fix: Cannot use the special principal ‘sa’. Microsoft SQL Server, Error: 15405
When importing a database in your SQL instance you would find yourself with Cannot use the special principal 'sa'. Microsoft SQL Server, Error: 15405 popping out when setting the sa user as the DBO of the database. To fix this, Open SQL Management Studio and Click New Query. Type:
USE mydatabase
exec sp_changedbowner 'sa', 'true'
Close the new query and after viewing the security of the sa, you will find that that sa is the DBO of the database. (14444)
Source: http://www.noelpulis.com/fix-cannot-use-the-special-principal-sa-microsoft-sql-server-error-15405/