Unable to connect to new Realm world

It's a new realm, just subscribed. Never was able to connect, neither on the win11 PC I created it with nor on a Nintendo Switch.

I get the message "Unable to connect to world."

I can download the hosted worlds and play them locally.

I have Microsoft Accounts.

I can connect to public servers.

I can join private online games, hosted by a friend.

I reset the worlds on the realm, closed and opened the realm. Used different world types, nothing helps. No resourcepacks are used, vanilla bed rock edition.