So I did some more digging and turns out this was all due to my having botched the migration. I had used the experimental upgrader script, which kept failing due to timeouts, so I went ahead and tried to finish it up manually. With everything else apart from the Android app working as expected, I was fooled into thinking I had nailed it. But with this issue persisting, I just had to check, and wouldn’t you know, there were still all these files lingering from the previous Owncloud installation that I hadn’t replaced.

Tl;dr; re-downloaded 9.0.52, replaced the existing installation with the files extracted from the download, and the Android app now also connects without issue.

@HucSte, I’m not sure your issue is related, but if you also migrated from Owncloud, you could try re-installing Nextcloud files afresh.