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