Thank you for your bug report. Our developers fixed that immediately. The fix is now under test, and I will let you know once it is publicly available.
After installing Buster Lite Image I made manual install of owserver.
sudo apt install owserer
During that install I got the same error messages.
Reboot and install rex with unipi script.
Everything was installed without any error.
Raspi is up and running without any issues.