Trevor,
Another simple scenario to check is to make sure the case of the password entered is correct.
Additionally, you can try enabling security access logging on the SOM machine to see if there is any error message logged in the windows event log?
1. Goto start-> control panel -> administrative tools -> local security policy
2. click on local policies - audit policy
3. Right click on "audit object access", select properties.
4. check the box to audit for "failure" attempts
5. Right click on "audit privilege use", select properties.
6. check the box to audit for "failure" attempts
7. click OK and save the settings
8. Attempt to login to REST admin using the domain user. If the login fails, there could be "audit failure" events logged in the windows event view security log.
