Comment 4 for bug 1668349

I'm unable to reproduce this with snapd 2.22.6 (not sure where you got 2.22.7) on series 16 with kernel 4.4.0-64.85-generic. It works fine logged into a console, in unity7 in a gnome terminal (under the terminal, screen, tmux and byobu) and in ssh. Are you able to reproduce reliably? If so, can you add this to /etc/apparmor.d/usr.lib.snapd.snap-confine:

/dev/tty rw,

then do:
$ sudo apparmor_parser -r /etc/apparmor.d/usr.lib.snapd.snap-confine

Then run the command and report back any stdout/stderr from the invocation? (I suspect something else is failing and snap-confine may be trying to display something somewhere)