Hi Axiome,
Thanks for the detailed bug report!
I am aware of the problem and working on a hotfix, but until then my advice would be to downgrade to Anki 2.1.26 if you can. 2.1.26 currently is Anki’s stable release, and you should be able to download it by clicking on the second blue button on the download page here.
Some other add-ons (both mine and from other authors) also seem to no longer work on 2.1.28, so 2.1.26 is positioned a bit better in terms of add-on support at the moment. It usually takes a bit for all add-ons to adjust to new Anki releases, especially when they contain a lot of changes. So at least in Anki’s current development phase where many things are moving around under the hood, as an add-on user it’s best to stick to the stable releases when you can.
Important : If you do decide to switch back to 2.1.26, please make sure to downgrade your current collection first. To do so please go to the File>Switch Profile menu item, and click on “Downgrade & Quit”. If you skip this step, you may get an error message when opening your collection in an older Anki version, and you will need to return to this version, downgrade, then try again.
Sorry again about any inconvenience this might cause! The fix should be ready soon.
For what it’s worth: There also is an official support thread for the add-on here. Just thought I’d mention it for future reference as posting there will notify me directly and also make it easier for other users with the same issue to see my answers. Thanks!