I'm running into this same problem. Unfortunately, I don't have an option to go back to R20. The odd thing is, the jobs aren't error'ing out, they are just rendering like there were no X-particles in it. I also tried both internal and external cache.
Hoping there is a fix soon, I lose about 1/2 day on my workstation now having to render locally (Team Render isn't an option in our environment).