cannot create new world within existing Realm

Hi,

it is impossible to create a new world within the Realm: My son can play within the existing world in his Realm, but he cannot create a new world for it, while he could do this many times before. The Realm was created new for a year several weeks ago.

With his Nintendo Switch he signs in -> Minecraft v1.16.1 / xbox account -> create new world -> game settings: immediate respawn, keep inventory -> create on Realms -> choose a Realm Server --> chooses his Realm -> Replace World? Confirm -> Upload initialized --> immediately occurs: AN ERROR OCCURED - and I can only press the green ok button.
(In the German language version we used normally it showed as well: level-export started...ended: that seemed to be ok, but it was not shown in the english version).

Using different modes - survival - creative - doesn't make a change, still not working

Please help, thank you!
showfl