If I execute magit-status in certain repos I get the following error:
Not Found: "GET", "/repos/////issues", nil, nil, ((message . "Not Found") (documentation_url "https://developer.github.com/v3"))

The repository url returned by (magit-get "remote" (magithub-source--remote) "url") is git@github.com:spebern/Bailador

As soon as a repositry url does not end with ".git" it won't match the regex parsing that url, because
".git" is not optional.