I managed to get it going for now (Avast Premier. Did a clean install, worked fine. Switched off the machine, when after a while I switched it on again, there it was again the UI failed to load. Rebooted and so far it is working, but this is getting ridiculous and rather upsetting.
Now on top of that a minor question: How to deactivate the bothering promotional messages for other avast products in Avast Premier?