I was having a similar problem with a server that I fixed earlier today. I ended up replacing the network cards and all is fixed. We use it to vpn into a network and log into equipment at a remote site. I ended up having to physically go there because we could vpn in, but had trouble browsing or logging into the equipment. While there, I put myself on the network and could browse perfectly. I reconfigured the server, switching which nic was wan and which was lan. It still worked great there, but when a user vpn'd in, it still did not work. I replaced the nics and everything started working perfectly again.
Just my experience. YMMV.