When creating a world or trying to open an existing world I get this error, I tested on a clean minecraft installation, but when I add some mods the error came back

If a press the yes button minecraft crashes, if a press no it return to the main menu.

If it crashes, post the crash log.

I found a way to fix it, I create a world then I enter the file of the world, I copy level.dat and rename it to level.dat_old, it worked! I still don't know what's happening

 

edit: tried to enter the world again, still getting the message