Recently, the core snap was updated to allow mir-kiosk (and other Wayland daemons) to provide the Wayland interface on Ubuntu Classic. (C.f. Can a confined Wayland server snap work on Classic systems? (And also Core systems?))
That change landed on edge and has been working.
However, today I find that something has change and that the client snaps (e.g. mir-kiosk-apps) can no longer connect:
$ sudo dmesg | grep mir-kiosk-apps
[ 3075.475586] audit: type=1400 audit(1552394983.889:105): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=8929 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3075.721444] audit: type=1400 audit(1552394984.133:106): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=8981 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3140.546873] audit: type=1400 audit(1552395048.956:109): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=9266 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3140.725463] audit: type=1400 audit(1552395049.136:110): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=9318 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3140.977838] audit: type=1400 audit(1552395049.388:111): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=9370 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3141.227692] audit: type=1400 audit(1552395049.640:112): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=9422 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3141.474101] audit: type=1400 audit(1552395049.884:113): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=9474 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3249.478494] audit: type=1400 audit(1552395157.891:127): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=11623 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3339.321978] audit: type=1400 audit(1552395247.734:132): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=12020 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3339.734672] audit: type=1400 audit(1552395248.146:133): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=12072 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3339.978609] audit: type=1400 audit(1552395248.390:134): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=12124 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3340.231538] audit: type=1400 audit(1552395248.642:135): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=12176 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
[ 3340.474981] audit: type=1400 audit(1552395248.886:136): apparmor="DENIED" operation="connect" profile="snap.mir-kiosk-apps.mir-kiosk-app-daemon" name="/run/user/0/wayland-0" pid=12228 comm="photoviewer" requested_mask="wr" denied_mask="wr" fsuid=0 ouid=0
Installing mir-kiosk-apps with --devmode is a workaround
[edit]
An even better workaround is to remember to connect the wayland interface!