Troubleshooting error in execution of OSC_Preprocessing script

I am running Siril v1.2.6 and stacking for the first time. I created the required folders for darks, flats, biases and lights. The OSC_Preprocessing script terminates while stacking the darks. The log keeps saying “Opening image 20 failed / Stacking failed”. Just above the failure message it says “Processing all images in sequence (21)”. There are a total of 20 dark subs in the specified darks folder, so why does it seem to say there are 21?

Hello, there is a probably an extra file in the directory from a bad copy or a previous run, also remove the process directory before running again, this should fix it if nothing appears where it shouldn’t.