Well Dietmar, look here: https://forum.proxmox.com/threads/lxc-backup-randomly-hangs-at-suspend.25345/
This seems to be a real bug. Reported 28-12-2015 on this very list. No reaction of the Proxmox team. Very unusual I must agree. But still.
For the rest I'm quite happy to use the Turnkey LXC...