With the advent of week-end I had some free time to dwell on this problem. And I think that I have found the source. As I have a base model of MBA with limited storage capabilities, I originally decided to download Xcode archive to an external SSD (Samsung T-5) and to unpack it on the same drive. The storyboards issue persisted while launching the app both on the external and internal drives. So I moved everything I could to the external drive to free up some space and unpacked the archive on the internal SSD. For one, it took extremely little time, compared to previous experience and, what's more important, Xcode 13 beta launched without a hitch. I don't think that this bug is 'popular' as most people will download updated versions of Xcode, when time comes. And other beta enthusiasts know better than to tinker with Xcode files on other drives.