Annoyingly, seems to be Mastodon-specific - can't OAuth to GotoSocial and flat-out refuses to even consider an Akkoma instance. Let's hope they sort that out soon (since both of them support enough of the MastoAPI that other clients work fine.)
(I wish the FediActivityPub people had spec'd up an API to avoid this kind of multi-implementation-who-supports-what shambles.)
EDIT: Amusingly, I fixed the "cannot login to Pleroma" problem for `madon` this week - Pleroma/Akkoma require form data in the body, not the URL, for POST requests (which is fair since the HTML spec suggests this is the Right Way.)
