Hi folks,
I discovered what the problem was. When I check my list of registered light frames (example, DSC_0038_c_r.xisf) most of the had 283,136 KB as their file size, but some of them had only 8 KB as their file size. When I included any of the apparently "empty" files PI stopped and gave the error message. When I removed the files from my list and ran only the "full" ones all went well.
Best regards,
Rudy