Some web sites - particularly those that contain personal info they must keep secure, like banks and those with health information - block access from IP addresses they don't entirely trust. VPN nodes can fall into that category because hackers use VPNs to hide their location. So it could be that. Altho it's weird that you cannot even get to the home page...
I don't know anything about Avast's VPN, but if you can pick from a list of nodes, try one that is physically near you - you will appear to be about where they expect you to be, and you may get better performance that way, too. Or try several different nodes - one of them may work.
Also, try using what Firefox calls a "private" window and Chrome calls an "incognito" window - this presents your session to the web site with no cookies or saved data, which may be what the web site is looking at, and they decided that info conflicts with the VPN IP address...
Good luck
Dan
Dan