Cannot start Docker on Plesk server: [graphdriver] prior storage driver aufs failed: driver not supported

Symptoms

• Dist-upgrade was performed from Debian 8 to 9

• Docker containers cannot be started anymore:

  PLESK_ERROR: Error: Docker service is not available: Connection refused

• Docker daemon fails to start:

  # docker
  <...>
  ERRO[2019-06-01T10:54:16.728849955+02:00] AUFS was not found in /proc/filesystems storage-driver=aufs
  ERRO[2019-06-01T10:54:16.728884634+02:00] [graphdriver] prior storage driver aufs failed: driver not supported
  Error starting daemon: error initializing graphdriver: driver not supported

Cause

Docker uses AUFS filesystem if the /var/lib/docker/aufs directory is present, however the AUFS driver that provides such functionality was dropped in Debian 9.

Resolution

1. Connect to the server via SSH

2. Move the /var/lib/docker/aufs directory to a different location:

   # mv /var/lib/docker/aufs /root

   Warning: The containers will be removed.

3. Start the Docker daemon:

   # systemctl start docker