UBudgie freezes immediately after login

I used to run UBudgie up to 19.10 without any major hickups. Today, however, the desktop had become totally unresponsive after a reboot; it completely froze about 10 seconds after login. I tried all I could think off to no avail. Luckily, I realized that the standard Ubuntu desktop was not affected.
I moved all folders ~.config/budgie-{desktop,extras,welcome} out of the way and managed to get some kind of starter window showing the following error messages.


Sure enough the folder /usr/share/budgie-desktop did not contain much; unfortunately I did not write down what. It did exist, though. I reset the budgie-panel from the standard desktop but that had no positive effect on a successive login to Budgie.
I re-installed budgie-desktop-environment which provided the missing files but that did not solve the general problem. Therefore I removed all packages related to budgie and re-installed ubuntu-budgie-desktop. That did not help either. So I removed it all again and upgraded to 20.04. After that I re-installed ubuntu-budgie-desktop and the problem persists.
How can I restart from scratch with a functional Budgie desktop?