Upgrading fully up-to-date 5.21.5 running in latest VMware Fusion and Virtualbox to current 5.21.90 beta results in blank screen with visible mouse pointer after login in. Tried a few things, including creating a new user account and result is the same.

Then, downloaded latest testing ISO and live session also only goes to the blank screen with visible mouse pointer.

https://www.dropbox.com/s/yl8jg1pqd7rse3p/kde-5_21_90-vmware-fusion.mov?dl=0

Sorry about this, I think it's a bad build of kwayland-server in testing edition, try an update or the ISO which will be published shortly.
Bug is at https://bugs.kde.org/show_bug.cgi?id=437104

Thanks for that. Looks like latest iso works again.

Now my upgraded VM hasn't been so lucky yet. With all latest updates applied, I still have the black screen with the mouse pointer. Also tried once again to add a new user and same thing happens. BTW. Am mostly using plasma with x11. Did test with wayland on the upgraded VM and it also still failed, so I must be missing something else.
Interestingly enough I can send ctrl-alt-delete and I receive the logout dialog and am able to logout and get back to the sddm login screen.

Have now started with a new VM with the latest iso and am now in the process to set everything up so that I can eventually just 'trash' the old vm. Keeping it for now in case to help any further troubleshooting/investigation.

No change so far after the latest round of updates.

An interesting point is that if you just leave it alone, the screen will lock showing the background picture, date/time and password field. After unlocking it is back to the black screen and only the mouse visible.
Same for any new account created.

I am now working with the new VM instead and will keep this one around for a little longer to see if it gets somehow resolved before I will delete it.