By default teamviewer uses port 80 and 443 for outbound communications. This is a problem if you are running a webserver. In order to stop teamviewer using these ports, open options, Advanced and under Advanced network settings check on Don’t use incoming port 80 and 443 (recommended for web servers only)
I have come across this case a couple of times where for some reason after operating successfully for some days or weeks all port 80 traffic starts getting blocked by a 2wire router. This occurs when the router is connected to the computers through a switch, as opposed to all computers connecting directly to the.