Unable to log in after installing Big Sur Beta

I've installed the Big Sur Beta on a development VM (Parallels on a 10.15.5 host). Initially I upgraded from a Mojave base line. The installation all goes fine and I finally get to the log-in screen of Big Sur. I enter my password and things start whirring but after a minute or so the screen goes black then after another minute or so I'm back at the log in screen again. I've done the oblatory, wait awhile, retries and reboots but still the same result. I've also tried increasing disk size and number of CPU cores in the VM.
I repeated the process starting with a Catalina baseline and have finished up with exactly the same result.
I have a couple of thoughts. First is this a known problem and secondly is this due to testing on a VM rather than computer? I've not had this issue previously and run VM versions of macOS back to 10.9 for testing.