VPN encryption

I set up a VPN server behind a firewall and the client is using a DSL. The VPN does work, but when I put a checkmark in the “Require Data Encryption” box it will still connect, but I lose the ability to browse the network. If I keep the checkmark off I can browse. I was wondering if there is a port on the router, or a policy on the firewall that needs to be implemented. Ever experience this before???
Cliff