Found out that updating from 1.7.4 to the snapshot 14w02c corrupted the world.
Here is a link to the thread: http://www.minecraftforum.net/topic/2293765-snapshot-14w02c-removed-single-player-world/

It's easily fixed with McEdit but it would be nice if the transition from 1.7.4 to 1.8 goes a bit smother and not via McEdit if you get what I mean.