I’m extremely embarrased - seems like I had mixed up those numbers (4 digits) in the subdomain when entering the hostname in DAVdroid, while on my desktop system, I was always using the right one (thanks to the browser’s history, that is). I’m very sorry - it was a mere typo.
One minor suggestion, my client included “https://” when entering the hostname (as do a lot of people, I think), and the error message isn’t very specific. So could you either issue a more specific one or just automatically remove that part?