avast doesn't block, it monitors and detects virus infection and alerts you to it. Also if you are starting at a https: web page, avast doesn't monitor secure connections as they are encrypted. The Web Shield only monitors http content, so it is likely to be something like cookie blocking that is causing this.
What errors or warnings are you getting?
I have no problem getting into PayPal.
What is your OS?
What is your Firewall?
What is your browser?
Are you using a cookie blocking tool?