This problem occurs on older versions of Mendix and is related to high resolution screens only. The screens in Mendix are not rendered correctly. Hence, the part of the xpath editor is not shown. Try finding different screen settings in Windows (Right click on Desktop and select 'Display settings') to solve the problem. Or try to maximize the 'Retrieve objects' popup.
Very strange and have never seen this. I would suggest to file a bug report so someone at Mendix might explain this. I would redirect this support ticket to this forum post (and do file the screenshots in the support ticket).
This is a very weird problem, I can't explain it. Maybe you need to reinstall Windows or try a different Windows account.