NeoObserver

This happened to me once on a multi-night narrowband image. I was new to narrowband imaging, and did not know it my problems were from acquisition of lights, calibration frames, or how I used APP. It took a bit to work through, and what I found was that some of the light frame FITS files had been corrupted. First I deleted two light frames which were obviously bad from File Explorer and restarted APP. Then I found that another sub was being included that was quite misaligned from the others. I unchecked it in APP. After that, processing went as expected and all of the warnings went away. I have no idea how common this is, or if this might help you, but like you I could not find anything helpful and just tried to exclude possible causes. Anyway, I hope it helps in some way. The more I use it, the more I like APP. Sometimes the documentation could be improved, but using the program is a great step forward for me.