I got this error on [ ] when attempting to enable a custom field with a broken field formatter. When saving the form, it can't enable the field so it throws this and the field remains disabled.
When entering an external URL, it's required to provide protocol (https, http, etc.), which does not start with any of the options below (`/`, `?`, and `#`. These only seem to apply to internal paths. Aside from the expected `<` as well, for ``, skipping over that conversation, the error message doesn't indicate the reason for the form being rejected, which is that it is missing the `https`. Recommend changing the text to explain why an external link starting with www, for example, causes the error.
"Manually entered paths should start with one of the following characters: / ? #" could say something like "Manually entered internal paths must start with `/`, `?`, or `#`. External paths must begin with a protocol such as `https:`, `http`, `ftp`, etc."