Disconnecting Players with "Internal server Error"

For experimental purposes I changed the server jar from the latest stable craftbukkit build to FTB-Yogscraft pack, due to a number of people asking for the change. There was a tremendous amount of lag on the server, so I reverted back to craftbukkit, only to find that It disconnects players with "Internal Server Error"

^ It then floods the console with this error message, that I can't make sense of. It's denying connection to my server and I don't know what to do.

Do you think a wipe of the "player.dat" files would help my situation?

A friend suggested that it could be a "dodgy chunk", he suggested changing the world and so far it works again.