Since you are now able to view .png files, likely a server glitch and not something on your end. I'd keep an eye out tho.
Thx. In the meantime, I was able to
fix Avast Free 15.10.4.2233 under W10 and Avast program update was set to manual (see first 2 screenshots)!
During the whole evening I did nothing with the laptop! One or 2 hours ago, I moved the mouse and the laptop showed me, that Avast had upgraded to 19.3. (see last 2 screenshots)!!!
But the biggest surprise was, that the SmartScan didn't show a NW scan error (see last screenshot)!!!