Seems @rafaelsales might be correct, this bug is still occurring. It seems circle is attempting to call a URL that does not exist in the Github API.
It’s quite surprising that the settings user interface has been broken, and I assume also the API for such a long time without any action.
As per @melekes question is there a workaround or is the solution to delete and recreate the project, and can this even be done? This this ticket suggests we can’t.
Just noticed this is the similar issue again as a this ticket that I was having last year.