Incompatible with Gnome 43
On https://extensions.gnome.org/extension/97/coverflow-alt-tab/ the extension is currently marked as outdated.
@tybor, I actually don't see that it says it's outdated. Can you post a screenshot?
As far as I can say version 43 of Gnome is not available when asking for a specific compatibility
Btw, many kudos as it is a really good extension!

I followed the above suggestion, setting disable-extension-version-validation in org.gnome.shell dconf and it seems to work fine, so AFAICS there's just a "43" missing in metadata.json
Another way to enable it without setting that dconf setting is to edit the metadata.json file and add "43" to shell-version:
"shell-version": [
"42", "43"
],
metadata.json would be in ~/.local/share/gnome-shell/extensions/[email protected] if you installed for just your user accout, or /usr/share/gnome-shell/extensions/[email protected] if you installed for all users. Probably you installed for just your user account.