After uploading the world during the verification of the world the error code 504 occurs.

I downloaded a backup from my Java realms, when I want to upload it again it always fails at the verification process. The file size is approx. 2.5 GB. It is updated to the new version. I tried it after concluding an ‘optimise world’ process. It is completely fine in singleplayer, but after uploading some older chunks regenerate, but the entities e.g. item frames, horses stay and float in the air.