This is likely due to the /var/backups directory being a mount which simply doesn’t support the same permissions as the local filesystem.
I’ll look into skipping the sympl-filesystem-security updates in /var/backups if it’s mounted, or just provide an option to disable that alone.
In the meantime if you sudo touch /etc/sympl/do-not-secure, that will just disable sympl-filesystem-security, but you may want to temporarily remove it and run it periodically just to be sure things in the /srv directories get permissions set if there are any manual changes.