I'm having exactly the same issue on a different website. Interestingly, the problem for me is also happens with gif files (it happened once for png files too). Everything else is fine on the same domain, but the gifs are never.
I have tried on a clean installed Windows 10 and on a Windows 8.1 and it's always happens but only with Google Chrome.
The error is always the same too: ERR_SPDY_PROTOCOL_ERROR
I have also tried turning off the https scanning, but the result was the same. As soon as I disable the web shield feature, it works. After I re-enable it, it works for X hours, then the problem starts again. It started like a month or two ago. I'm using the latest avast Premier version (10.3.2225). I think it started with an earlier update.
Edit: A few plus info. I don't have comodo firewall, I use Google DNS too, but it's managed by the router, the PC gets the router's IP as a DNS server via DHCP.
I have also tried to play with the settings, but the only solution is to turn off the web shield component.
I have also read and experienced an issue on normal a normal webpage: the error showed up, then page refreshed (google chrome tries to load the content again if there is an issue), and the page loaded fine.
Edit 2: The issue is also represent on facebook for example. Again, the gifs...
Is there anyone from Avast who can help me/I can help him to debug this?