can't unmount openvz container

I have a Proxmox with 15 VM - OpenVZ Container.

Yesterday i try to delete 1 container and have this error:

"Can't umount /var/lib/vz/root/110: Device or resource busy
TASK ERROR: command 'vzctl umount 110' failed: exit code 51"

"Can't umount /var/lib/vz/root/115: Device or resource busy
TASK ERROR: command 'vzctl umount 115' failed: exit code 51"

When i try to unmount any container happend the same...

What can i do???