I got the file and I have confirmed that it was the cover jpg (600x800) that caused the hanging. So I think that there is a bug in the jpg handling for certain jpg. I have seen jpg 600x800 that works.
Code:
mobi2mobi infile.prc --outfile fixedfile.mobi --fiximagesizes
can maybe fix the problem. There was a bug in mobi2mobi so it did not work on Linux in certain cases. I had to add a sleep(1) in mobi2mobi before the call that read the file.