Hi again,
@Danika_Dakika I have followed your suggestion and disabled all addons - so far so good, the issue has not come up since. I also believe to have possibly identified the culprit: there is a “Ankizin Notetype” addon, which is a community-developed extension to the Ankizin deck. It features a lot of custom code and is not really well documented.
So for anybody facing similar issues: try disabling this addon. The Ankizin deck will work fine without it.
I will report back if the issue arises again. In any case: thanks a lot for your help!! 