EU Server
PC - Windows 10
So, this has happened on multiple occasions now where quest progress breaks when me and my friend are questing together.
Sometimes an NPC that is supposed to be talked to can only be spoken to by one of us and the other cannot, thus meaning they cannot progress further, other times it is objects that need to be picked up/placed, one of us will interact with the object and then the other is unable to.
Most recently this happened with the quest "The Unquiet Dead" in Grahtwood, when we progressed through the quest to the part where the Pale Sentinel posesses the dead soldier, after it is defeated and you use the bone staff on it it's supposed to become an NPC you talk to to continue the quest, as me and my friend approached it I was able to talk to it, he was not. We had to disband the group, he quit the game then logged back in and it was fixed. Then, just moments later in the same quest, when it came time to place the Skull, Gem and Essence in the appropriate locations when one of us placed an object that object was removed from both players inventory, but only activated for the one who placed it. And as I placed one and he placed the other two, neither of us could progress the quest any further, but the ghost was telling us we had everything we needed, but when we interacted with the shrines it didn't give us the options to place the remaining objects, nor could we go back to the original locations to pick them up again. And unfortunately this time, quitting the game and reloading didn't fix the issue, so we had to abandon the quest and start it again from the beginning, thus meaning we had 2 quest breaking bugs in the same quest line, one of which we couldn't even bypass.
Later in the same session (we were only playing for about 1 - 2 hours) I had a similar bug of not being able to interact with an NPC in the "Eyes of Azura" quest where I couldn't speak with the quest giver to hand it in and had to repeat the whole quest.
Unfortunately this is just the latest incident in a long line of similar occurances, I've lost count of how many times this has happened already just in the last month since we came back to the game. If this continues we'll be forced to basically play the game solo just to make sure we can progress the quests, at which point there would be no point in us playing together at all.