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
source & further reading
gist.github.com — original article
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