AppImageKit forces the Appstream metadata to be validated, but appimageutil in Ubuntu is way too old and fails on valid AppStream metadata. Since it will take over 4 years for the update to land in Ubuntu, I think the best way to deal with this is to just build a patched AppImageKit version with the check disabled; We just download a prebuild AppImage of it anyway.
AppImageKit forces the Appstream metadata to be validated, but appimageutil in Ubuntu is way too old and fails on valid AppStream metadata. Since it will take over 4 years for the update to land in Ubuntu, I think the best way to deal with this is to just build a patched AppImageKit version with the check disabled; We just download a prebuild AppImage of it anyway.