Where do you see this error message? Take a look at the error log in the hosting. The cause should be visible there.
WordPrerss Dashboard -> Tools -> Site Health
Loopback requests are used to run scheduled events, and are also used by the built-in editors for themes and plugins to verify code stability.
The loopback request returned an unexpected http status code, 400, it was not possible to determine if this will prevent features from working as expected.
Unfortunately, it is difficult to recognize in your posts what are quotes and what are not. The best way to do this is to use the Quote block, see: https://ww.wp.xz.cn/support/forum-user-guide/block-editor/#quote-block
Have you already had a look at the error log? The actual cause should be recognizable there.
I see a lot of messages, but nothing that could be the direct cause. I would recommend that you contact your hoster’s support team, as I suspect a misconfiguration in their systems.
Ok, I will talk to them. Thanks for your help.
