After booting from a snapper read-only snapshot I tried to execute a 'snapper
rollback $SNAPSHOT_ID'.

When I try to do that after booting it complains that

  Creating read-only snaphshot of current system. IO Error.

Running snapper list shows that I have a no snapshots created ( only
'current' one is available ) and indeed /.snapshots is empty.

This worked once I mounted the .snapshots subvolume:

  mount -t btrfs -o subvol=.snapshots /dev/sda1 /.snapshots

This subvolume should IMO be present in /etc/fstab by default, otherwise
recovery is much harder.