Been getting this since yesterday. Same for /spawn. Can still warp though.

This is a known problem a lot of people are having right now. I believe the potential FTBM map reset is related to it. Just going to have to wait for it to be fixed, but (I assume) it's being worked on already.

It's a permissions issue. FTBm permissions are still working but Vanilla seems broken. Broonie is aware of the issue and will be attempting to solve it.

http://crazy-fools.co.uk/p/forum/forum_viewtopic.php?39319.0#post_39364