OK, it looks like snap-update-ns makes some mistakes when freezing the cgroup, and ends up freezing itself. Can you please paste here the output of /proc/<PID>/cgroup, where PID is the process id of:
hello-world-
snap-update-ns(you might have several of these ones running; try figuring out which one is the correct one, based on its parent PID, which should be the one ofhello-world)