Just had this same problem.
SNAPCRAFT_CONTAINER_BUILDS=1 snapcraft clean
snapcraft --version snapcraft, version 2.40.1
snap version snap 2.32.1 snapd 2.32.1 series 16 ubuntu 17.10 kernel 4.13.0-37-generic
performing the clean also fixed the problem for me.
Just had this same problem.
SNAPCRAFT_ CONTAINER_ BUILDS= 1 snapcraft clean
snapcraft --version
snapcraft, version 2.40.1
snap version
snap 2.32.1
snapd 2.32.1
series 16
ubuntu 17.10
kernel 4.13.0-37-generic
performing the clean also fixed the problem for me.