I thought it might be "Enable web interface" option that I've always had turned off. I turned it on and the error went away. Someone should document that this option must be enabled, assuming I found the answer.
I now get "NextPVR connection to tuner lost" in XBMC. I have both the 12.0 beta 1 XBMC, the Dec 1 pvr.nextpvr.zip, and NextPVR 2.5.9 with the patched Nshared.dll.
NPVR 3.2.9 o XBMC 12.3 o EventGhost 0.4.1.r1640 o SchedulesDirect o SAF 6.2.2
Case: Apevia X-Qpack HTPC o Motherboard: Asus P8H67-MLE o CPU: Intel Core i3-2100 o RAM: 8 GB o OS: Win7 64-bit
Tuner: HDHomeRun dual tuner o Amplified Leaf antenna o Remote: Microsoft MCE
Hard drives: Western Digital 1 TB SATA2, Seagate 2 TB SATA2, Samsung 540 GB SATA2 o Input: Logitech USB keyboard & mouse
Video: ATI Radeon 7750 o Monitor: Viewsonic VX2703MH-LED, Vizio 47" LCD TV