I think I may have solved the problem.
It seems that upon reboot the AvastUI.exe starts in unprivileged mode (i.e. non-Administrator) thus it can't communicate with AvastSvc.exe properly.
So basically what I did was:
- Turn off self-defense option.
- End AvastUI.exe process.
- Set privilege level to Administrator in AvastUI.exe compatibility tab.
- Run AvastUI.exe.
But I have a feeling that it will now show UAC prompt every time I boot so this issue should still be considered as a potential bug.