After today's update to 24.04 LTS, thumbnails no longer being created

 Steps to fix it:

1. Relax the security restriction (immediate fix): Open your terminal and run: sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0

2. Clear the "failed" thumbnail cache: This forces Nautilus to retry generating thumbnails for files that previously failed: rm -rf ~/.cache/thumbnails/fail/*

3. Restart the file manager: Close all Nautilus instances to apply the changes: nautilus -q

source : https://www.reddit.com/r/Ubuntu/comments/1sij087/after_todays_update_to_2404_lts_thumbnails_no/