500 error on Nextcloud 14

[anarcat]

this remote completely fails to transfer a file on a Nextcloud 14 server, hosted in a subdirectory (example.net/nextcloud/).

$ USERNAME=anarcat PASSWORD=REDACTED git annex initremote owncloud type=external externaltype=owncloud encryption=none folder=git-annex url="https://example.net/nextcloud/remote.php/webdav/"
initremote owncloud  13:48:39 [owncloudannex-0.2.1] startRemote : '['EXTENSIONS', 'INFO']'
 13:48:39 [owncloudannex-0.2.1] startRemote : '['EXPORTSUPPORTED']'
 13:48:39 [owncloudannex-0.2.1] startRemote : '['EXTENSIONS', 'INFO']'
ok
(recording state in git...)
$ git annex copy --to owncloud
 13:48:45 [owncloudannex-0.2.1] startRemote : '['EXTENSIONS', 'INFO']'
 13:48:45 [owncloudannex-0.2.1] startRemote : '['GETAVAILABILITY']'
copy test.txt (to owncloud...) 
100%  5 B                10 B/s 0s 13:48:47 [owncloudannex-0.2.1] postFile : 'Failure: '' - 500'

  File could not be stored
                                  
100%  5 B                15 B/s 0s 13:48:48 [owncloudannex-0.2.1] postFile : 'Failure: '' - 500'

  File could not be stored
failed                            
(recording state in git...)
git-annex: copy: 1 failed

I don't know if it's relevant that the server is hosted in a subdirectory, but I suspect it's the prime reason I'm also having trouble making the remote work with the normal WebDAV support in git-annex support itself (reported here).