I am having the exact same problem. I have searched everywhere in the forum and can't find anything.
I have three computers, one of which I use to play from my stereo. They are all on a network, so I copy my music directory from my main laptop to the others, but the directory/file structure is slightly different on each machine.
After doing a Library > Backup on my main computer I have tried copying the entire JRiver directory directly to the other two machines and I have also tried just copying the most recent backup.zip file. Both yield the same results... the files won't play.
I have also tried re-importing on the other two machines and then restoring the libraries, but that doesn't work either. This used to work when I first started using MJ12, but now that I've purchased MC13 I can't seem to get it to work anymore.
I thought it was supposed to be easy to maintain the same music and libraries on multiple machines.
Can someone please shed some light on this?