My old account got banned by Discord (yay!) a few weeks ago, and I ended up with a bunch of …s displaying under my Favourites that I couldn't click on nor remove, even though I've re-added the same servers / friends again. Ended up doing this:

$ sqlite3 user_data.ripdb
sqlite> delete from playlist;

I'm guessing Discord channel_ids for the same channel / friend differ between accounts? If that's the case you should see the same thing when logged in to a different Discord account.

(Not sure about Slack; I didn't have any favourited at that point, nor do I think so from what I remember of their API.)