When I hover over a mob that I need to kill for a quest, it doesn't tell me this or anything else related to the quest (like the amount or the name of the quest) anymore. Also when I hover over quest items in my bag it doesn't tell me which quest exactly they are related to. Yesterday everything was ok, what has happened?

Try Reset Questie in Questie Options > Advanced

Thanks, that did the trick. At least with new quest items I gain, this didn't change anything for the quest items I already owned, but that's alright. This also solved the issue with the nameplates, thanks again