DavidR
19
My suggestion to use the link should have gotten around any hosts file blocking of the messenger.yahoo.com sub-domain. This is a common malware trick (though not great) to block sites, but that is normally to stop you accessing security sites. So I think it unlikely that it would be malware as I wouldn’t have thought it would target messenger.yahoo.com, but be more widespread.
Sorry I’m at a loss as to what else to suggest as all of this appears to be outside of avast.
I couldn’t even find any support links in Yahoo.com, so all I can suggest is try some google searches and see if this is a problem others are having.