Yeah, the debug trace shows that the node had an ID of 0. How can this happen (since the node has been functioning well for months before)?
I flashed the EEPROM and reinstalled the sketch, after which I was able to include it with a valid node ID.
After looking at the flood of packets more closely, it turns out it was a combination of factors: a faulty IR sensor in the lock caused rapid flipping of the sensor's state, and the flood control timeout was set to 0...
