When Active Directory groups were removed and re-added in ADMIN->Security->External Group Mapping page, Existing external user account (without domain) gets authentication error while logging in to NetWitness GUI.
However, UserPrincipleName (USER@DOMAIN.COM) able to login GUI.
This issue is due to existing external user account conflict.
When external user first time logs in to GUI, useraccount will be created automatically in ADMIN->Security->Users page. When Active Directory groups removed and re-added. The user account should be recreated during the first login.
Please follow the below steps to allow User (without domain) to login GUI.
Navigate to ADMIN->Security->Users page.
Select all existing old external user accounts (without domain) and delete them.
Verify GUI login with the user account (without domain) which will be a success.
Note: User custom dashboards still exist as earlier access.