Hi Dan -

If the "registration.gz" file is missing, then there is no transformation stored in your output directory, which would certainly explain why you cannot read it.

So really, the question is, why the files aren't written sometimes. I assume you're not out of disk space? (Sorry, I have to ask)

The only reason I can think of why the registration.gz file isn't written is that the tool that produces it (warpx?) crashes either during registration or during file writing. Do you have any outputs logged from the commands that produces incomplete outputs?

Best,
  Torsten