The history so far is as follows.

July 12 night
Multiplayer under the same Wi-Fi is no longer possible in my friend's world (it was possible until the day before).
It does not proceed from the loading state of "Creating the world and searching the server".
If you cancel the reading, "○○ has left" is displayed in the other party's world.
Friends can participate in my world.
July 13
Chat consultation on the above matters.
I was offered to restart the router, so I implemented it, but it didn't improve.
Chat consultation again.
Application version, Microsoft account verification, Microsoft account registration age verification (all are fine).
Receive a proposal to test in the following four-step flow.
① Ask a friend to create a new world under the Wi-Fi environment you are currently using and check if you can participate.
② Check if you can participate in the problematic world under a Wi-Fi environment other than the one you are currently using.
③ Ask a friend to create a new world under a Wi-Fi environment other than the one you are currently using and check if you can participate.
④ If none of them work, contact support
After implementing ①, I was able to participate.
However, I couldn't participate in the copy world of the problematic world.

July 14
I chatted again about the flow so far and downloaded the world data registered on the Realms server before (although I do not have a contract now) because my friend's currently stored world data may be corrupted. I tried it, but when I was loading it, I got a message saying "The connection was lost" and I could not connect.
I restarted the router, but the situation does not change.
I copied the downloaded Realms server data, but I can't put it either.
I tried to make a new world with my friends that was made until the day before, but I can't put it in either.

It is a flow.
I haven't verified some items yet, but maybe my application data is corrupted.
When I was playing solo in my world, I had the above problem after several crashes in the Nethergate test.
Please let me know if there is a way to correct it while keeping the current world data.

Thank you.