I get this every time I try anything using /. It works fine on servers, but on singleplayer, it fails. D: It won't let me save my world either.

The other instances all seemed to involve multiplayer/servers. I'm just trying to play on a self-created, saveable world. Is this something I can fix or is it due to the instability issue I read about? D: If so, does that mean it might work better at a different point in time? o.o

Thanks in advance and sorry if this clutters the forum. >.>;