Make sure the user checkbox for enabling the user stuck upon save (seems to be a programming bug with that screen). Log in as the admin user on another workstation to verify that the checkbox stayed checked.
If the checkbox is not checked on another workstation and the single user mode seems to have gone away, reboot all the workstations and the server to clear up locks and try again.
Seems like the cheaper accounting package was not as cheap when you figure downtime for bad programming causing downtime in your company eh?