It isn't just what you are currently using, if you have had a previous AV that can have an impact if remnants of it are left behind. Even if there were remnants, which could cause a conflict, it is unlikely that they would present themselves like that, e.g. a 404 error.
A 404 error isn't avast as if it were blocking youtube (which it doesn't as it just scans and alerts to infection) then you wouldn't even get a 404 error.
Can you give an example of a link you can't access and I can try that link.
So I'm at a bit of a lost as to what is going on.