Steps to fix it:
Relax the security restriction (immediate fix): Open your terminal and run:
sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0
Clear the "failed" thumbnail cache: This forces Nautilus to retry generating thumbnails for files that previously failed:
rm -rf ~/.cache/thumbnails/fail/*
Restart the file manager: Close all Nautilus instances to apply the changes:
nautilus -q
Single Line Command:
sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0 && rm -rf ~/.cache/thumbnails/fail/* && nautilus -q
This is actually driving me nuts. As someone who works with lots and lots of graphics, my workflow has been massively affected.