Great progress!
Likely caused by sandboxing parameters in onion-grater’s systemd unit file. Similar to this platform specific issue:
Try commenting out this line:
https://github.com/Whonix/onion-grater/blob/master/lib/systemd/system/onion-grater.service#L54
After changing that file:
sudo systemctl daemon-reload && sudo systemctl restart onion-grater && sudo systemctl --no-pager status onion-grater
If that doesn’t help, try out commenting out all onion-grater systemd sandboxing.