chgrp failures when deploying repository

I have a repository hosted on an external EXFAT-formatted disk that is mounted inside an Orange PI board running Ubuntu (Armbian).

When I execute frdoid deploy the rsync call fails with multiple errors regarding setting file permissions:

2023-01-15 13:15:42,599 INFO: rsyncing repo to 10.1.1.15:/var/www/html/homeshare/android/fdroid/
rsync: [generator] chgrp "/var/www/html/homeshare/android/fdroid/repo" failed: Operation not permitted (1)
...
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1333) [sender=3.2.3]
2023-01-15 13:15:49,981 CRITICAL: fdroidserver.exception

and the repo index file is not created as a result.

fdroidserver version: 2.1-1 (installed via WSL on Windows)

Could you add a way to pass additional options to rsync to ignore permissions?