Proxy or VPN behind the router

I have a home network behind a router, and I’m running a proxy server with ports forwarded through the router. Some clients connect to the Internet via my proxy, but there's a small problem — sometimes they cannot send files through email or HTML forms (I’m using Squid as the proxy). I tried to set up a PPTP VPN using PoPToP, but when connecting through my router, I encountered error 619. The server is running Fedora 16. What is the best way to provide Internet access to the clients — using a VPN, a different proxy configuration, or something else?