Singleplayer-World Upload ”An Error occurred”

Minecraft Bedrock 26.3, PS5, Realm-Server: Netherlands; Location: Germany, Realm-Storage: 10GB
When Uploading a Singleplayer World (370MB) to my Realms there is always an Error.
”An Error occurred”

The World:
Created: ~1.17.0 and Updated every Update to the newest Version.
PS4 Version: The World is 930MB on the Singleplayer-World-Menu and 370,80MB on the Realm Upload.
PS5 Version: The World is 370.80MB on the Singleplayer-World-Menu and 370,80MB on the Realm Upload.

Things i tried to get i to work:

I restart every time i try an new Upload.
I tried the PS4 and PS5 Version of the game. (Controller is Plugged in, so no connection stop)
I opened the Ports 25565 and 19132-33.
I tried Uploading with Lan and WLan.
My Internet speed is 250MB/s Down; 50MB/s Up, the Upload to the Realms is ~0,1MB/s.
My Internet Provider checked if he is the Problem, he is not.

Changing Realm-Server-Locations doesn’t work, because only the NL server is able to connect to my Realms.
I tried all 3 realm Slots
I cleared all Data from the Realms so there is nothing left in the Storages.
I tried replacing the Worlds, Uploading directly from the Singleplayer-World Menu after and before resetting the realm world.

What I did:

The Error occurres either while Uploading the World or immediately after the upload is finished.
There is never the “scanning” screen after Uploading, the Error is always before.
I’m trying since 1 Week ~ 5 times a day (cause the Upload is for some reason so slow)
The World never goes beyond the Upload and is never playable on a Realm.

My Guess:

As there is such a difference in the size of the world and the Upload on the PS4 Version, i think it’s in connection with the Map bug.
If you Create a Map the size of the World increases a lot, maybe this process creates a lot of very small files and these Bug out the Upload and the scan after the Upload.
The Error while Uploading could be a Internet problem
The Upload speed could be the many very small files slowing it down
The Error directly after the Upload finishes could also be caused by the many files freak out the file scan.