Hi,
you gave the right hint, but i had to config this:
acl caldav-endpoint path_beg /.well-known/caldav
http-request set-path /remote.php/dav if caldav-endpoint
use_backend nextcloud-http if caldav-endpoint
acl carddav-endpoint path_beg /.well-known/carddav
http-request set-path /remote.php/dav if carddav-endpoint
use_backend nextcloud-http if carddav-endpoint