Unhelpful and suspect error "instance failed to match at least one required schema among 2"

The UI is simply displaying that error - google suggests that the payload carrying that error has significantly more detail, except the UI is not rendering any of it: there needs to be a way to see the entire error, not just this small description.

And, on the subject of the error itself: it suddenly started today, I don't think it's correct - all my backed-up YML produces the same error now, but none of them had errors when I backed them up last week.

... and on the subject of the error wording: that is a totally useless description. I have no idea whatsoever what it might mean, or what it's referring to, or why. This is a textbook example of "how never to write an error message". Please find what issued that error, and adjust it to include something useful that we can find/reference/understand!

The issue names no files, tests, or entry points. Trace where the schema error is generated and where the UI renders it; done means exposing the full available error detail and replacing the vague message with information that identifies the cause and reference.