Solution from https://forum.proxmox.com/threads/minor-apparmor-problem-with-tor.173419/ and https://pve.proxmox.com/wiki/Roadmap#9.0-known-issues
Add "abi <abi/3.0>," to the beginning of "/etc/apparmor/init/network-interface-security/usr.sbin.ntpd", then "systemctl reload-or-restart...