I think this is an easy one, very kind of you to let me warm up first 
Are these super long files? Assuming you are running defaults (2 components?) then these are one minute sound files (10mb per min * 2channels). If you can open these sound files by dropping them back into REAPER then its likely the process is just taking a long time and not finishing in a reasonable timeframe for you. NMF is generally a super chunky algorithm to run and its complexity spirals with the fft size and the number of iterations.
Can you try this for me:
- Small sound file 2-10 seconds
- Iterations 50
fftsettings 1024 512 1024
and see if it runs without locking up forever?
If you are able to send me the source media that you are testing and the parameters I’ll be able to rule out a machine specific error too but I think its more likely that you are just asking the algos to work through a lot of numbers.