Xbox 360 and One worlds fails to load

There is a very big problem with my save from Xbox 360, that was created the day Minecraft came out on Xbox.

Me and 2 of my friends played this world since the launch of Minecraft on Xbox 360, and we still use it today... so this world is now very big, there are constructions everywhere etc.

So to resume the problem :

i imported this world from 360 to One a long time ago, and at this time, the transfert was working well. the world is now bigger, etc. i can load it on my Xbox One, it works fine.

BUT : i tried to import it on the Minecraft Bedrock Beta on Xbox One, and the game freeze at 90% of the tranfert. Everytime.

so i wanted to try something : reimport the world from 360 to One : and here it is :
On 360, the game freeze when loading the world. Impossible to launch my world. So i tried to upload it to One and see if my One can load it.

after the transfer, i launch the world on One : the game freeze exactly like my 360 when loading the world. So i guess last updates broke something on loading old worlds. Because before, i was able to load this world on 360, One, etc.

Now the only thing i can do is play the world we used until now on my One, but i can't :
-play it on 360
-load it on Bedrock Beta version
-load the freshly imported world from 360 to One

a bug could come from my world, since it was created the game was launched... i can send the 360 world to you if it can help