if you start MM as admin then the [Install Service] with install the service with the admin account (i.e. admin password on the Log On tab)
if you start MM normally (from the user account) then you need to enter password of the user account, and the same password is used on the "Log On" tab.
So I guess that previously you either mistyped the password or there was the password mismatch re admin/user account?
To confirm could you start MM normally (user account) and click [Uninstall service] then click [Install Service] and type the correct _user_ password.
I guess this works?