Any further updates on this?
I've just got a late 2011 C-Max, and while almost all the mp3's I'm playing off a 16GB Sandisk USB stick work fine, some of the audiobooks give the 'invalid file format' message. No part-playing then stopping. Some work, some don't.
The only difference I can tell is that the stuff that works is music or speech ripped from CD's, whereas the four audiobooks that don't work were bought online and downloaded as mp3's. The audiobooks that were ripped and work are at 128 kbps bit rate and 44 kHz sample rate, and those that don't are at 64 kbps and 44 kHz respectively - but I can't see why this should make any difference for the file format validity, just the output quality.
Any ideas? Many thanks!