@patdavid And since I'm doing all this testing of alternative software, why not a Raw Therapee version as well.
Although, I can't seem to find the spot removal tool (note, not googling for any of this testing, just seeing how intuitive the interfaces are).
@gemlog @dadegroot My first instinct here is to say that the old version of darktable doesn't support the camera that the raw file came from (it's been the solution in the past many times). So the best first guess is to get a recent darktable and try with that instead.
If that fails we can start dissecting what the root cause might be. :)