It is a generation error, I'm afraid the best way to fix this is to just regen a world
I believe you have a client side mod installed that is trying to control minecraft's main menu screen, make sure there isn't a client side GUI controlling mod. (Ex. CMM)
I found that conclusion from line 192; "java.lang.NoClassDefFoundError: net/minecraft/client/gui/GuiButton"
Can you provide anymore info? Crash reports and logs would be appreciated and crucial to finding the problem.