Netgear CG3000DCR stopped answering DHCPv6

Overnight, the cable modem has stopped answering DHCPv6. This means that, once my lease expired, my downstream router dropped the delegated prefix and now my local hosts can no longer communicate via IPv6. This is causing a lot of local networking problems.

I have tried rebooting the cable modem. No change. Snooping traffic on the wire shows that my local router sends DHCPv6 requests, but the Netgear cable modem never responds. The "Enable DHCPv6" checkbox is checked, and the cable modem does show both an IPv6 address and a /56 IPv6 prefix (the same one it's had for quite a while, now).

Before I rebooted the router this time, it was showing a couple weeks of uptime, and it had been answering DHCPv6 requests all that time. So it doesn't seem like a bad config pushed to the cable modem, if my understanding is correct that it would have to reboot for that to take effect.

Vendor Name: Netgear

Hardware Version: 1.04

Firmware Version: V3.01.15

Operating Mode: Residential Gateway

What has gone wrong, here? What can I do to restore service?

Thanks,

- Geoff