Errors when bouncing in Surround 5.1 ITU

Hi folks, first post so apologies if not in the right place.

 

I'm converting some tracks from stereo to 5.1 ITU, using Nugen Halo Upmix. (Yes, I realise it's not a true surround but this is what's required for this project)

It's working great except that when I go to bounce the splits there seem to be errors - If I try to import those splits into a new Logic session to listen back, the R channel constantly flags as 'file can't be found' even though it's sitting there. In addition, the file extension it says it can't find is actually different to the one I'm trying to import! I've tried the process from scratch and double double checked my settings in Logic but no change.

 

Any help much appreciated!

 

Using Logic Pro X 10.2.0, Digidesign 002 rack (old but dependable), and latest demo version of Nugen Halo Upmix (full functionality)

Macbook Pro Retina 2014 10.9.5 / 2.4 Intel Core i5 / 8GB RAM

Update: on closer inspection it seems there is no audio in the 'R' track, but all others are there. They were all bounced together as splits, so I don't understand why one of the six would screw up every time. It's always 'R' that goes wrong. So confused!