Unable to progress past Greybeards.
Our skyrim save broke once we climbed the mountain to the Greybeards. All they say is "sky above, voice within", and I can't actually start the questline with them. I don't see any workaround for it either, loading into single player doesn't fix it. The save is now broken. I can load a save from before starting the quest to go up the mountain and complete it in singleplayer, but this issue should be be addressed.
For heavily scripted scenes, I believe it's recommended you do them on single-player, then re-connect and sync up once you're on less script-heavy quests.
Well, I would consider this an open issue until there's either safeguard to warn/prevent the player from doing a quest/area that's heavily scripted, or until the issue is fully resolved and players can play all quest with other players.
I will say, if anyone else has a bugged save and isn't able to progress the greybeards, you can fix it in singleplayer by using this console command. "setstage MQ105 30"