@Tony I tried that without any success, so I went ahead and put a new router in and it fixed the issue. I'll get an RMA going on the old one. Thanks for your help.

@Tony PPTP passthrough is enabled, and the PPTP connection is being initiated by devices behind the router, not the router itself. I was suspicious of the ISP changing something on their end, but...

The ability to connect to my workplace PPTP VPN stopped working yesterday even though I haven't made any changes to the router. The VPN connects fine with the computer plugged directly to my cable...