I looked at it today and it appears correctly.
Maybe it was the cache server.

I have closed. The cause of this issue was the URL, not the Extension.

Thank you for your research. This URL can be displayed as plain text in the browser. I don't know but maybe it requires a special method to connection.

The problem occurred again. I don't know how long it has been occurring.
It seems to depend on the MIME-type on the destination side.
I have tried several RSS feeds and servers with MIME-type of "text/xml" seem to be able to read them correctly.

I found the cause of the problem. It affects Extension:ForcePreview if it is installed.
I uninstalled ForcePreview and the problem was solved.
Thank you for your patience.

In T8568#173177, @Universal_Omega wrote:

Hello, which page gives you the error? If multiple then just one example. Thanks!

We've solved it. Thank you very much for investigating the cause of this problem.