Microsoft Dynamics GP Deleting User Error “Deleting the login failed for an unknown reason. Contact your SQL Server administrator for assistance.”
Every time I have encountered this is when a user is set up to a specific company database. To resolve this follow these steps:

1. Go to your SQL and verify that the user has a SQL login setup. If they do then follow
2. Expand your company database
3. Expand Security
4. Expand Users
5. Right-click on the user you are trying to delete and select Delete. This will remove the user’s SQL profile from the database and any schema’s that are causing the error.
6. Repeat for all company databases
7. Repeat in the Dynamics database

NOTE: DO NOT REMOVE THE OVERALL SQL LOGIN only under each specific database
Log back into Microsoft Dynamics GP and remove the user through the normal user setup. This will remove the user from Microsoft Dynamics GP and also SQL.