I had this annoying error when trying to add new database diagram to SQL Server 2005 i received the error
Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.
And even when i tried what the error message suggests didn't work.. i Google the issue i found many others suffering from the same and even when i tried the solutions introduced and never worked for me also reseting the compatibility for the database one step back to SQL Server 2000.. this how this error resolved which is itried before via Managment Studio but didn't work only did from a SQL Quary.
EXEC sp_dbcmptlevel 'Dashboard', '90';
ALTER AUTHORIZATION ON DATABASE::Dashboard TO sa
So keep in mind if you ever ran into this.