Near the original spawn point, blocks become unbreakable and chests won't open. The glitch point started small and is getting bigger.

I started a realm and invited my nephew to play, but after about a month and a half, this glitch began to be a problem but only on their system. I tried to reproduce the effect, but wasn't able to find it and all his chests opened for me. The only difference I found was that I'm running on Vista, and he's running on XP.