If you log onto the application as SA and then attempt to remove SA from the Activity table, then go back to the Application you will encounter these 'weird errors'
Start by logging off of GP
Open SQL Server Studio Manager and open the Dynamics..Activity table.
Note if SA is still there - if so, ensure that there are no other users or sessions open under the SA user ID and then and only then, using SQL, remove that ID from the table.
Best practices with SSSM and changes to the database is to ensure that you have a good backup and understanding of what effect the adjustment in SQL will have to the application.
Hope this helps. Keep posting