The workaround described by @unknown worked for me:
Archive Minecraft on your Switch - this will delete Minecraft, but keep your data and worlds
Re-download Minecraft - this will download a fresh installation and restore your game data
Go to your realm settings, tap replace and select the world you want to upload.
It will then upload the world, write something about initializing and finally post a note that it was unable to verify the successful upload.
It then took a few tries (connection unsuccessful) until I could log into my realm, but finally, my world was there. Yay!
Chris Et
The workaround described by @unknown worked for me:
Archive Minecraft on your Switch - this will delete Minecraft, but keep your data and worlds
Re-download Minecraft - this will download a fresh installation and restore your game data
Go to your realm settings, tap replace and select the world you want to upload.
It will then upload the world, write something about initializing and finally post a note that it was unable to verify the successful upload.
It then took a few tries (connection unsuccessful) until I could log into my realm, but finally, my world was there. Yay!