printing handle is invalid

OnEndPrint() doesn't do anything but directly call Windows' EndDoc() API function.  Pointing at a problem with the print subsystem of Windows.  Technically, it would be possible for your PrintPage event handler to be causing this problem, when it allocates a lot of pens, fonts or brushes whose Dispose() method you don't call.  Make sure you use the using statement in your code.

---

Hans Passant.