I'm also having exactly the same issue. However, I downloaded the sample application and implemented the Comparer code in exactly the same way (I copied and pasted it) and I still have the same problem with one particular set of data. The custom comparer code we have, has been in place for nearly 3 years and this is the first time we have encountered this issue. We're using RadScheduler and there are 17 appointments for the same day. If I change one of the appointments so that the start date lands on a different day, it works fine. Just like in the post above, the calendar for the current month is initially shown without any problem. However, if the user then clicks to navigate forward or back one month, I get the same error referenced above. I even hard-coded the Compare method to always return 0 as a test and that didn't work either. I also tried removing the custom comparer altogether and it seems that whatever default sorting that RadScheduler does causes the same exception to be thrown but on the initial load so that even the current month's calendar will not be shown.
Is anyone else running into this?