The x-default link is the same in all the pages and it is present in all of them.
That would seem to be "the problem".
The "Missing confirmation link" error is because the
http://example.com/language page (source of
x-default) does not contain a corresponding
hreflang link back to the "about" page(s). The circular reference is not complete. If you are using the same
x-default page for all your pages then this is clearly not possible.
In fact, Screaming Frog replied in a comment to a blog post:
Google describe it in their hreflang guidelines here – https://support.google.com/webmasters/answer/189077?hl=en
“Missing confirmation links: If page A links to page B, page B must link back to page A. If this is not the case for all pages that use hreflang annotations, those annotations may be ignored or not interpreted correctly.”
I have not seen anything that would suggest that
x-default should be treated any differently to any other language
hreflang value in terms of creating the appropriate confirmation/back link.
As I understand it, the
x-default link should be used to refer to a default landing page or homepage on a multilingual site, not necessarily for every site page, unless you had a suitable default for every page. In your example you would need a specific default for the "about" page. Maybe you could do this by displaying the (default) "about" page with a more prominent language selection which could perhaps be accessible via a URL of the form
Otherwise... under what circumstances do you think Google would return a page that is only for language selection when the user has supposedly searched for and found the "about" page?
Personally, I don't think Google would even return the language selection page in the SERPs for such a search. If they did and the user was expecting an "about" page then that would be a bad user experience.