I installed a new 18.04 VM and I can reproduce this issue there.
The folder /snap/gnome-3-28-1804/current/usr/share/mime on VM contains much less data comparing to my working machine.
However, after updating the system through Software Updater and restart, Opera is finally running if I update the system through Software Updater, and manually connect gnome-3-28-1804 interface:
opera@opera-VirtualBox:~$ snap connections opera-developer
Interface Plug Slot Notes
audio-playback opera-developer:audio-playback :audio-playback -
avahi-observe opera-developer:avahi-observe - -
browser-support opera-developer:browser-sandbox :browser-support -
camera opera-developer:camera - -
content[chromium-ffmpeg-93464] opera-developer:chromium-ffmpeg-93464 chromium-ffmpeg:chromium-ffmpeg-93464 -
content[gnome-3-28-1804] opera-developer:gnome-3-28-1804 gnome-3-28-1804:gnome-3-28-1804 manual
content[gtk-3-themes] opera-developer:gtk-3-themes gtk-common-themes:gtk-3-themes manual
content[icon-themes] opera-developer:icon-themes gtk-common-themes:icon-themes manual
content[sound-themes] opera-developer:sound-themes gtk-common-themes:sound-themes manual
cups-control opera-developer:cups-control - -
dbus - opera-developer:dbus-daemon -
desktop opera-developer:desktop :desktop -
desktop-legacy opera-developer:desktop-legacy :desktop-legacy -
gsettings opera-developer:gsettings :gsettings -
home opera-developer:home :home -
mount-observe opera-developer:mount-observe - -
network opera-developer:network :network -
network-control opera-developer:network-control - -
network-manager opera-developer:network-manager - -
opengl opera-developer:opengl :opengl -
password-manager-service opera-developer:password-manager-service - -
process-control opera-developer:process-control - -
pulseaudio opera-developer:pulseaudio :pulseaudio -
screen-inhibit-control opera-developer:screen-inhibit-control :screen-inhibit-control -
unity7 opera-developer:unity7 :unity7 -
upower-observe opera-developer:upower-observe :upower-observe -
wayland opera-developer:wayland :wayland -
x11 opera-developer:x11 :x11 -
I opened a new post to request auto-connect for gnome-3-28-1804.
Do you have any thoughts about the system update part?