I am having same problem,
first post here, please let me know if I do something wrong 
(opensuse tumbleweed, nextcloud 16.0.5, apache2-2.4.41 php-fpm 7.3.10)
Logins from the desktop client and android app both worked prior to switching from php7 to php7-fpm.
(switch was required to enable http2)
Nextcloud web interface shows the client as connected, the client shows as logged in, but the clients (tried on multiple machines) cannot sync. The android client briefly showed the same 401 error as noted above.
Android app offered “old method” login but that failed as well.
error follows
[PHP] Error: Undefined index: urlGenerator at /srv/http/nextcloud/core/templates/loginflowv2/done.php#29
POST /nextcloud/index.php/login/v2/grant
from 192.168.1.xxx by xxx at 2019-10-22T16:07:02+00:00
Did you have any luck resolving your issue?