Why can't I join his Bedrock world?

Today I (pc player) wanted to join my friend (Xbox player) to his Minecraft Bedrock world, but it didn't work. However, he can join my worlds & we can also use public servers or realms. If I want to join him, it just says "World is being created - server is being searched for". And that all the time.

What can you do about it?

Maybe you will learn German properly first.