Yes I get this problem all the time. I believe this happens when a frame has been deleted or more then 1 animation has been applied to the skeleton. I have found a way around this, when I'm finish replacing my animation I straight away try to load the .ifp up again (and with success it loads with no error messages) which means its good to go into the game for use but when "unknown property count in undefined" pops up I close and reload IFP IO. copy my back up working .ifp file over the corrupt one and then open it up in Max again. I replace the file and it works this time. so it seems like the first time (while editing the animation) information gets mixed up. reload the IO program with a working .ifp file and it should work.
bugs: I have notice while in game that the animation works fine but other animations such has shot or death animation the bones collapse in on them self. I'm not sure if this is related but I believe so.
so to sum it up try loading the IFP IO up again and apply the animation to a new working IFP file.
hope this helps, let us know the out come