I also had this behaviour. Sometimes certain web sites stop being accessible. I had it with OpenBC.com and an other site which I don't remember. Weird enough: I even could not ping the site, the IP was completely blocked. When I closed Avast and restarted it, everything was fine again.
(It was not OpenBCs fault, because I checked their server from another PC and found that it was working fine, it was just my own PC which couldn't access it.)
It doesn't happen often, but sometimes.
Solutions for me:
1. close and restart of avast
2. wait some minutes, it tends to fix itself after a while
Looks like a bug to me
Otherwise its a fine piece of software, do like it.
Regards
Daniel