Strangely I've started having the same problem as Tim this week, beside regular upgrade I did reboot since last backup... (Ubuntu LTS 16.04)
if I stop docker, the backup will proceed, otherwise i get the same failed to freeze / error message, even with no container running, just the daemon. That's both true with 1.11.2 and 1.12.1 docker dameon version.
Strangely I've started having the same problem as Tim this week, beside regular upgrade I did reboot since last backup... (Ubuntu LTS 16.04)
if I stop docker, the backup will proceed, otherwise i get the same failed to freeze / error message, even with no container running, just the daemon. That's both true with 1.11.2 and 1.12.1 docker dameon version.