Hi FDC
If You can log in with manage /!manage credentiqls then just finish the wizard restart the storage few times
then try to add some new users
DON`T delete / disable manage user yet
If You can create new users try to log-in/off few times - restart the storage again and try again
And then you can disable manage user.
If You can`t login with manage user just execute the steps i have posted to restore the defaults and manage user is back again.
Good luck.