There is available lxc-cgroup, but maybe your issue is related to systemd in lxc is not able to safely move userspace processes (like mount) due missing/wrong rights on host (but this is reported for kernel 5.5.x after upgrade from 5.4.x , TOS5 has 4.4.x right ?).
It seems that Debian Buster is now using apparmor, causing some issues to services managed by systemd (apache2,mariadb,mount…)
Enabling nesting for the container will fix this issue for now, until we get an upstream fix.
lxc.apparmor.profile = generated
lxc.apparmor.allow_nesting = 1
I am not using nextcloud anymore, so this is wild hint. but i hope it will help 
EDIT: maybe just reboot is needed …