Put the summary of the bug you're having here

What I expected to happen was...:
I would join my world and continue building on it. I was building this world since over a year and never encountered any problems.

What actually happened was...:
The whole world got reset. It was kind of like joining a new world with the same seed with the exception that I could keep my inventory.

Steps to Reproduce:
1. Start a new world
2. Build on it for a long time... 
3. Join it one day
4. Its gone.