The add-on suddenly stopped working (2.1.53, 2.1.54)

[jcznk]

Hello, first, thank you for the very useful add-on.

As in the title, the add-on suddenly and quite mysteriously stopped functioning.
I tried the following: I checked the database; disabled all other add-ons; restored a back-up of the add-on I made a few days ago (when the plugin still worked); I created a new profile, then deleted and re-downloaded the add-on, then restarted Anki; restarted my computer.
Nothing worked. The symbol replacement only appears to work in the browser search bar.

I do not know what might have gone wrong. The only unusual thing I did was that I opened Anki 2.1.22 to check something. I also did not change the addon folder first:
I have a few addon folders, one for each major version. If I am now using Anki 2.1.54 and I want to switch to Anki 2.1.22 I would usually do the following, but this time I forgot to:

1. close Anki
2. rename addons21 to addons21.54
3. rename addons21.22 to addons21
4. open Anki 2.1.22

(The same addon version I downloaded using Anki 2.1.54, and which does not work in Anki 2.1.54, nor 2.1.53, seems to work fine in Anki 2.1.22)
I doubt it has something to do with it, but I really have no clue on what else might be the cause.

Thank you in advance

[jcznk]

UPDATE: sometimes the add-on works, sometimes it does not. I have yet to understand which triggers one or the other.
(e.g. I have two Add windows open at the moment. In one it works, in the other it does not)

Some debug info: