dd-wrt refusing new configuration: out of NVRAM space

Another post documenting a small issue just so I can google it later: one of the Cisco/Linksys WRT160NL devices (both running DD-WRT) in my environment stopped accepting config changes through its web interface. It happened to be the one I'm using as a NAT gateway and DHCP server, not the one just functioning as access point.
A config backup appeared to be full of UPnP rules. Those are port forwarding rules that have been asked for by applications on NATed stations that want to be reachable. Think Skype amongst many others.
Clearing the UPnP list solved the problem: click "NAT/QoS", then "UPnP", then "Delete all". Apparently the UPnP list filled up the available NVRAM space, which broke all subsequent config changes.

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the birdhouse in my soul

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