@Volvstang 

If nothing changed on the router end, then something changed on the opposite end. 

Go to Advanced > NAT Forwarding > ALG, and make sure PPTP passthrough is enabled. 

If that still does not work, create the PPTP connection on the actual device rather than the router which should then work.

@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 when a computer is plugged directly into the cable modem the VPN connects without issue.

Today I popped in a switch between the cable modem and the router and used a second laptop to run a Wireshark on the LAN and WAN sides of the router simultaneously and it appears the the GRE packets are not making through the router. Without a firmware change, I have no idea what could have caused this to stop working.

@Volvstang 

If possible, you could try factory resetting the router (hold reset for 15 seconds), then reflash the current latest firmware if the config file somehow changed or got corrupted.

@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.