That log shows an attempt with the 'DivX H.264 Decoder', which will definitely not work. Unfortunately H.264 decoders are not really interchangeable, and I'd only expect a very small set to work. Can you post the logs again, but with the ArcSoft decoder?
Thanks for the pointer - and I've had some success already. Although I had 'a' ArcSoft decoder installed, I've downloaded the ArcSoft H.264 Decoder v2.21.31.21 again and lo and behold it worked right away (both NPVR and GBPVR). I assumed my original install was corrupted. Then I found a newer version (ArcSoft H.264/VC-1/MPEG2 Decoder v2.27.249.90) and expected this to be better/more stable. To my surprise I was back at square 1: Audio but no video.
Lesson learned: don't give up on the first version you get hold of.
Now on to my next challenge: ITV HD which won't play at all after recording - not even in any other player.
I've ended up using the MS decoder for H264 as it was the only one I could get to play nicely with BBC HD since it went VBR. Works fine for ITV HD too.