I have a 508, which is basically the same model with a smaller hard drive. A similar thing happened on mine recently when I changed the smart card. For some reason all of the timers were changed to the point where they found wrong information (or no information) on the guide. So far the fix has been to do a smart card pull reset, then a check switch. The DVR then downloads the guide again and most of the timers are OK. Some I still had to delete and re-enter.
Personally I am wondering if Dish is introducing these bugs intentionally in order to get people to upgrade to a new DVR.