I experienced this same problem earlier today, but in Windows 7.
I booted up my computer this morning and logged into my general personal use profile. I had stepped away to start making some coffee at that moment, so didn't get to see what was happening. I came back to find an Avast window taking up my screen, telling me that Avast had updated and needed a restart to take effect. Instead of doing all of that I clicked "restart later" and closed the window. That was when I noticed that the desktop appeared to as if it were a default desktop for a newly created profile.
Thinking I had accidentally logged into the wrong profile I logged out and logged back in again. That was when I noticed the message about the temporary profile. Baffled, and mostly in desperation, I restarted my computer. As it in the process of rebooting it occurred to me that whatever was happening might be related to the Avast update. Upon reboot, I was able to log in to my profile without any issue.
While the problem is easily fixed, it's still extremely troubling. I can't help but wonder if Avast has purposely caused this problem to force. First of all, I didn't initiate an update, and my update preferences are generally supposed to require that I choose if/when to update the program. That leads me to suspect that this was somehow a forced update that overrode the user settings, and basically was designed to lock users out of their devices before continuing further, kind of like how Microsoft force updated some people to Win10 against their will. Second, if not an intentional design, then this kind of bug raises severe concerns that Avast updates might accidentally inflict irreversible damage to users' files. Which would really defeat the purpose of having an anti-virus program in the first place.
What I know is that if I encounter this kind of behavior again with Avast, it will be the last time.
