In case this is the same problem as Snap-update-ns failing, cannot launch snaps, this may be useful:
13:58 ~ $ snap version
snap 2.39.2
snapd 2.39.2
series 16
ubuntu 18.04
kernel 4.15.0-51-generic
13:58 ~ $ snap debug sandbox-features
apparmor: kernel:caps kernel:dbus kernel:domain kernel:file kernel:mount kernel:namespaces kernel:network kernel:policy kernel:ptrace kernel:query kernel:rlimit kernel:signal parser:unsafe policy:default support-level:full
confinement-options: classic devmode strict
dbus: mediated-bus-access
kmod: mediated-modprobe
mount: freezer-cgroup-v1 layouts mount-namespace per-snap-persistency per-snap-profiles per-snap-updates per-snap-user-profiles stale-base-invalidation
seccomp: bpf-argument-filtering kernel:allow kernel:errno kernel:kill_process kernel:kill_thread kernel:log kernel:trace kernel:trap
udev: device-cgroup-v1 tagging