So, I've just hit a bug I've never encountered before. When Isran arrives in the cathedral after Harkon's death, he mentions something about Harkon being dead and starts to say his dialogue to Serana, but barely a word into it he cuts out and just stands there with his mouth open. He's impossible to communicate with. He isn't entirely frozen, since he still looks at me and Serana and has the idle sway animation. But activating him does absolutely nothing. I've tried reloading to before entering the castle and before returning to Fort Dawnguard with Auriel's Bow, but it keeps happening. He also won't leave Castle Volkihar, I fast traveled back to Fort Dawnguard and a few other places and he never leaves the Volkihar cathedral.

Which basically means I can't do any other side quests involving him, like finding that priest of Arkay. Kinda game breaking, and I have no idea why it's happening or how to fix it. Anyone have any thoughts?