can't start game since 1.7.4. using single player,

version 1.7.2 ran fine, problem started as soonas update to 1.7.4 was completed. cannot open a world. while building world, game crashes and says "connection lost, internal exception: java.lang.classcastexception:bju cannot be cast to fm." I updated Java, and no change. no other changes to computer have been made. Does not generate a crash report, so I can't attach one...