Yeah last month this happened to me too.
I didn't understand what happened at the time. I ran the clean up, it got stuck during phase two. I didn't give a crap and resumed with my work. I restarted my PC and I couldn't log in.
After restoring my profile, which was mainly due to sheer luck, I uninstalled Avast along with other programs I thought to be the culprit. I wasn't 100% sure though. Everything went back to normal.
Anyways after realizing how much of a crap Windows Defender is I reinstalled Avast. Today I ran the clean up and it got stuck at stage 2(registry optimization). I panciked and searched google and I came across a couple of topics of people complaining that Avast corrupted their profile. Then it became clear to me that Avast responsible for messing up my PC the last time.
I went back to my restore point(luckily Avast created one before the system clean up) and everything seems to be OK. But I'm not sure if I want this thing in my PC at this point. Yes I can always skip that registry step, but then I paid for it in the first place. In the previous update(when the interface was different) that option seemed to be working just fine.
Anyways, Is there any chance we can get a fix ?