Lee,
Thanks very much for the info. I took your lead and used the iPhone app and the default service time/interval and was able to reset the reminder. Now at least I don't need to look at the "SERVICE" warning all the time while riding. I then went in and changed the date and interval to a non-default value and the controller rejected it just as before, so the same result you got. Clearly the problem isn't with our units but rather something in the software.
I hope Hexcode address this as the GS-911 provides precious little functionality other than resetting the service reminders, and doing so with intervals of your choosing. At best it is a read-only device for error codes otherwise and a questionable value proposition.