Uh... I've just found something weird, in addition to the previous.
Each character Hotspot has that Dialogue Actor script (I don't think that was there before 2.x), pointing to a stream of Hex characters.
I thought that could be the problem, but the error persists after changing that nonsense to the appropriate character name from the dropdown list.
Now I'm wondering if I can delete those 'Dialogue Actor' scripts safely. Because I don't really know why they're even there, in the AC Hotspot object.
Edit: this is what I've had in 1.8.6, instead of the Dialogue Actor script:
And I don't even know why I had those 'Override Actor Name' components there, lol. If there was a reason, I forgot long time ago. But the thing is, they've turned into a different script with 2.x upgrade.
I guess this is not related to my original problem, right?