What is it that they have running at fullscreen mode (F11) ?

I have just tested this with just my browser (and avast settings ‘Silent if a full-screen application is running’ option enabled:
a) Maximum icon, see image.
b) using the F11 function key to have it at full screen.

In both cases cases the avast screensaver started as it should and started scanning my system.

So I don’t believe it is related to this ‘Silent if a full-screen application is running’ option but something else the “Failed to initialize” can also be related to other scans. If it were truly a bug I would expect this to be more widespread and I don’t see that in the forums.

Try a repair of avast. Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow.