Starting yesterday, my game crashes whenever Serana is engaged in combat (no issues when she is just following around). She's been my follower since the start of Dawnguard and I've never had any crashes previously.

Using the same bandit camp, I did some tests by dismissing her and clearing out the camp alone - no crashes. I reloaded and brought Lydia along - no crashes. Then I brought in Serana and the game crashed (always happens within 30 seconds of starting the fight). I've disabled my mods and the game still crashes.

I have the papyrus logs enabled and I noticed alot of "DLC1SeranolevelingScript" errors. Anyone have insights into this problem?

System specs:
i5-3450
GTX560
8GB ram

Well, I did notice a couple of crashes from time to time whenever Serana is fighting beside me, it's true.
But I'm running the Unofficial Dawnguard patch and it's not a huge problem - it's fairly rare.
Plus, having just pretty well finished with Dawnguard (after ~44 hours) I can say that it got better over time...
Huge fights in big crowds can cause crashes anyway.
But Serana dual-wields two magic attacks and is hard on the computer, re-animating dead guys etc. (Lots of visual effects.)
Install the UDGP right away - I recommend and endorse it whole-heartedly!
Regards

I've installed both the unofficial skyrim and dawnguard patch, but it still crashes everytime she fights. I also have a bug where Serana cannot be injured; she does not even react to attacks on her (similar to http://www.youtube.com/watch?v=Aym1Hc2_58k)

Anyway I've since created a new character - after a few hours of her fighting, no crashes happened. There must be something triggering CTDs on my older character