Apologies for the extra notification, but the command:
rm -rf ~/.cache/gstreamer-1.0/
instantly resolved my issue. In that directory was a file
"registry.x86_64.bin" and after deleting it everything works perfectly
as before. Totem, gnome-sushi previews, everything.
The libsodium / PPA issue I had was probably related to this bug, but
none of the workarounds posted there resolved video playback issues (nor
did removing all PPAs and their related packages):
** Bug watch added: github.com/keepassxreboot/keepassxc/issues #1545
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Some libraries that the gst-libav package relies on are missing.
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