The web shield used the C:\WINDOWS\Temp\_avast4_ folder to scan http traffic coming from the browser, before it is allowed into the browsers cache, so it can be displayed.
So if this _avast4_ folder is either corrupted or unreadable then the scan couldn't take place nor the pass through to the browser cache so it can be displayed.
If this were to happen again:
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. This has in the past resolved this out of sync issue between reported and actual VPS version.
Or
A temporary solution would be to pause or terminate the web shield so traffic isn't passed through/scanned (means your protection isn't as good).
Or
Disconnect from the internet, and try deleting the _avast4_ folder and reconnect, I believe that should recreate the _avast4_ folder.