system
1
The connection was reset when open setup page of OMRON MR504DV VPN ROUTER.
100% reproducoble.
Uninstall(using avastclear.exe) and re-install 17.3.2290 does not help.
Disable Web Shield fixes the problem.
Avast 17.3.2288 was working without such problem.
(Windows10 Home 64bit 1607.)
alanb
2
I can confirm that this is an issue since 2290 was installed (but not before).
The two sites affected for me I have been visiting for years, and they are famously trustworthy (Downloadcrew.uk and mediamonkey).
The problem seems to be that Avast has decided the “offending” sites are portscanning (Check Firewall Logs > Rules).
Adding the sites to Exclusions does not work. A workaround is to switch off portscanning detection (uncheck: Settings > Components > Firewall > Customize > Advanced > PORT SCAN DETECTION). I, for one, would rather be able to have this enabled 
system
3
I am using Windows Firewall.
So your problem is different.
system
4
Finally, Avast Beta 17.5.2300 fixes this problem. 