in BetterQuest\modules\sink\sink.lua line 73 you have the following code:
self:Pour(ERR_QUEST_COMPLETE_S:format(title), core:GetColorFromCompletion(1))

this :Pour function is LibSink-2.0's embedded function which has the following call signature:
Pour(addon, text, r, g, b, font, size, outline, sticky, loc, icon)

The problem is now that core:GetColorFromCompletion(1) has (besides other) the following code to return values:
return unpack(self.db.profile.colors.done

where self.db.profile.colors.done is a table containing 4 values (RGBA) instead of just the expected 3 (RGB). This results in :Pour(...) being called with an unintended value for 'font' which can result in errors.

In my case, using MSBT, it resulted in an error with LibSharedMedia-3.0 due to LibSink-2.0 trying to register a font using the passed name.

Making sure that only three values are returned by core:GetColorFromCompletion(...) should fix that.