This is happening with me too. I get this error on my own realm and on all of my friends realms.
I have 3 realms and I got this error in all of them.
I got the same error on all of my devices (xbox one x and android)
I get the error when I:
-Replace the world of the realm
-Download the realm world
-download a backup
But I can join the realm and replace the world with a backup.
Obs: I tried to buy a new realm, and when I tried to upload my world to the realm i bought, I got the same error.
I have this issue when I try to join my own realms and all of my friend's realms.