World didnt open

Our one world (size 1.4GB) can no longer be opened and if I then press Okay (see following appendix) I really cannot open or join another world, let alone create a new world. I then have to restart the entire game. The world that unfortunately can no longer be opened has already gone through several versions, but it is just 1 year old and the file for the world is still undamaged and completely available. I also don't know what triggered the whole thing. I left the world normally by saving, then closing the game and the next day I couldn't open the world anymore.