In Database view : 

1. Go to Administration 

2. Select Set up system security

3. Open the affected user

4. Select the Signature Security tab

5. Unmark all of the signatures, then Save and close the user.

6. Have the user sign out of NXT

7. Open the user back up

8. Go to the signature security tab and mark the signatures needed
9. Save and close. Then have the user login and try again