When you remove computers from the LanGuard dashboard, LanGuard fails to delete a computer. It is trying for a couple of minutes but afterward fails without an error, and the machine is still there.
The behavior described points to an issue with the backend scan results database since this is where the computer data is saved. There may be problems connecting to the database, or it may have run out of space and got deadlocked.
Please review and follow the recommendations on Maintaining the SQL Database used by LanGuard.
Try to remove the computer again. If the issue persists, contact GFI LanGuard Support.