Will update this as I find more bugs and potential solutions and causes.
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Multiplayer Desync.In regards to the desync issue. At first tested on a friend hosting from US, big desync issues as soon as combat starts. Assumed it was just the connection. Hosted myself from the UK second, worked fine, for a good while. Reach the bandits in the temple ruin, desync begins after initiating combat while I host. We close the session and relaunch and initiate combat without dialogue. Problem is gone. Friend talks to the guy behind the door and then we head down the hole and initiate combat. Desync issues immediately.
Probable cause and possible temporary solution - When my friend left the session I remained in it, and immediately desync issues were gone and I could control everyone again. But curiously enough when initiative passed onto my friend's character the hotbar disappeared and the UI for a dialogue opened up, with the buttons in the bottom right. I clicked the dialogue history to find the dialogue for the man behind the door outside the temple. I could not exit the dialogue UI and was effectively locked out of continuing the fight.
I believe this means that the main cause of issue isn't from combat desyncing, but dialogue. Often times one player is ahead of the other in dialogue if there is skipping involved, and when you quickly skip entire dialogue segments the issues begin. Like seeing the dialogue box above your co-op partners head which the game won't allow you to save because of (this is solved by entering a new dialogue and leaving it.)
The solution is likely for the time being to take dialogue slow, so that it doesn't desync. Need more testing on wherever it's safer to be inside it or outside it when someone else is talking.
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AI attacking each other.This occured in the druid settlement. After a fight broke out between the druids and the tieflings, I noticed towards the end that Zavlor had accidentally hit another tiefling. This caused all tieflings around him to become aggressive, and promptly formed a coup against him. Haven't seen this happen again, so perhaps a rare bug. As a side note, once leading the goblin attack to the village and arriving late, I found that I could not speak to the Drow in Zavlor's room. She was there, could be attacked, and spoke to herself, but was unclickable and so I could not finish the quest.