Whenever i enter a world it freezes

Put the summary of the bug you're having here
It Crashes whenever I enter a world it says : Expected BEGIN_ARRAY but was STRING at line 1 column 1.
What I expected to happen was...:
Describe what you thought should happen here
I thought I should enter my world without it freezing. In 1.8.3 it crashes as soon as you start to boot up the game with the same error
What actually happened was...:
Describe what happened here
It Crashed
Steps to Reproduce:
1. Try to enter a world
2. Crash your game
3. Tada