So today I connected my M10 directly to my computer using a stereo 3.5 to stereo 3.5 cable.
I recorded for approximately 5 hours and didn't notice any drops on the right channel.
Since that eliminated the M10 line-in input, I went back and connected the original rig to try and replicate the problem.
I have tried to recreate the problem for a few hours but without luck.
The only differences I have made to the rig since it failed mid-concert was to change between the capsules and then going back to the original setup and I also changed the order of the 9v batteries in the nbox.
I thought that this may mean that I had some connection issue and one of the batteries in the nbox wasn't connected properly.
On the other hand, this doesn't really fit with the fact that when the issue occurred, moving the interconnect cable from the line-in input to the mic-in and then back to the line-in solved the problem.
I wrote Nick and he also suggested that this sounds like an interconnect problem.
I will try to cause the failure again, but in case I am unable to replicate it, I guess I will have to wait until the next concert and see how it behaves in a concert environment (in terms of high sound levels and handling of the rig during a concert).