Chunks at 0 and teleportation back to start

Sorry in advance, my English isn't the Best.

My Version is updated to the newest version, but i like version 1.5.1 better, that is the reason i am playing in that version.

Only one of my Worlds is affected with this bug, so my Computer isn't the issue.

But i do not want to delete this World, because i made many things on this hardcore world.

Is there an opportunity to play on this world again?

Now to my problem:

My Problem Started with a round about 30 to 30 block with a full height not loading at all.

The Rest was loading without problems.

Because that wasn't really a problem, i played further.

While that was happening i could play normally  for a few hours.

After that the real problems started.

I could only mine sometimes (some pop right back) and to pick the drops up took really long.

To counter that i played in another world and then the world with the problems functioned probably again for a few minutes.

After i did that a few Times it Didn't function anymore.  

While loading the World it takes much longer than usual and after it loads i find myself right besides a cliff that is leading to nothing.

This cliff is on every side of my round about 30x30 Block i am standing on with probably the full height.

When i go over the cliff i fall very slow like i am in water and i can even go back into a cafe further below. I haven't tried to fall completely down, because i am playing Hardcore.

While that is happening i can go around and mine but i wont get any drops and after some time i will teleport back to my start point.

I will also teleport back to my start point, when i reload the game.

Most of the time the Chunks are at 0 and even if i changed the settings, the chunks would go right back to zero.

While i tried to start the game again and again i got a Crash report, but i could start the world again after that. I probably got that Crash report, because my Computer loaded an update at the same time and it was to lame to load it. But i will still include the Crash report.

I also included a few screenshots from my problem.

I also included a forceful crash from myself from 08.09.21.