How to fix broken Vampire Lord without starting over

I'm experiencing an issue where after Harkon's Gift, in the tutorial room, I can transform into a Vampire Lord, but I can't land or use the left or right hand attacks. Additionally, after turning into a vampire in front of Harkin when he asks me to, he yells "Where'd you come from?!" and shows a health bar like an enemy, although he doesn't actually attack me.

I've been searching online and can only find old posts from around 2012, and no real answers.

Some have said its mod related, some say its caused by using showracemenu, some says it only ever works if your character is a certain race.

I'm assuming that because no one is still talking about it, that it has been fixed, but I'm not finding the solution.

Is there a console command that can fix this? What type(s) of mods would cause the problem? Are there certain ones with known issues?

I do have the unofficial Dawnguard patch installed, and have tried various things like restoring to the castle entrance auto save and trying again several times, and also trying while naked to see if it was equipment related.

What else can I try?

Update 1

I've discovered that I cannot progress by refusing Harkon either. When you refuse, he says, "I banish you" and then does nothing. He just stands there. You can't leave or talk to anyone.

I have also tried removing all mods and I encounter the same problem.