Processing image with TIFF flats

Hello,
I have four sets of images:

  • biases (ARW)
  • flats (TIFF converted from JPG - I forgot to set RAW format on my camera)
  • lights
  • darks

I managed to make stacked dark and stacked flat, but I have problem with lights processing:
preprocess light_ -dark=…/darks/dark_stacked -flat=…/flats/pp_flat_stacked -cfa -equalize_cfa -debayer -stretch
22:51:39: Running command: preprocess
22:51:39: Reading FITS: file light_00001.fit, 1 layer(s), 6024x4024 pixels
22:51:39: Reading FITS: file dark_stacked, 1 layer(s), 6024x4024 pixels
22:51:39: Reading FITS: file pp_flat_stacked, 3 layer(s), 6024x4024 pixels
22:51:39: Preprocessing…
22:51:39: Error: statistics computation failed.
22:51:39: Preparing sequence processing failed.
22:51:39: Script execution failed.

My hunch is that it’s a problem with demosaicing (RGB layers for flats, one layer for the rest). I would appreciate any help on how to deal with this problem.

Your flat is probably demosaiced and in 8bit. Siril will not take it. It is not a good idea to use it.