chromium-browser: ERROR: not connected to the gnome-3-28-1804 content interface
Solution 1:
OK I found this solution on the snapcraft.io forum
snap connect chromium:gnome-3-28-1804 gnome-3-28-1804:gnome-3-28-1804
doesn't output anything but after I ran it, chromium ran again happily.
I'd love to know why it got broken tho!