I found that disabling my bashed and tes5merged patches worked, if I load a game before getting the mysterious note. If I load my game after that, because I had done a lot of side quests, then the console commands successfully advanced the quest and it seems to be working fine. If I have my bashed and tes5merged patches enabled, then even console commands fail. Perhaps I've merged or patched something that messes everything up. I haven't seen this mentioned, so hopefully this helps someone else. This is using Skryim SE, with about 130 mods.