Most problems (program crashes) are related to bugs which I fixed a long time ago. You are using a very old version (0.8.16), because newer versions are not compatible with Python 3.9 on MacOS.
But even that seems not to explain the file datatype problem reported in dark file processing. Did you try my suggestion to move the .tiff files only into a new directory and use that one for the dark file processing?
I hope I found the reason, the images were converted as a batch with Canon's DPP 4 to a 16 bit Tiff file, when I opened the image in GIMP, a thumbnail is also part of the file. I imported as layer and only selected the page 1 and then exported to a new directory. I repeated this 5x's for each Darks, Lights and Flats. I was able to stack in PSS using these images. Now I have to find out why the Darks didn't match the Flats and was deactivated. I also have to find a way to batch convert the raw images to TIFF without a thumbnail, or start shooting videos.