Thank you very much for the continued testing, you are most helpful. This is exactly what I want to do now, figure out any issues and solve them.
An issue that I think you had in trying to solve these images is that you used the default options. The thing is that I am trying to develop different options profiles to be used for different purposes. This library is effective for both Sextraction and Solving, but the desirable option settings for each task are very different. For source extraction to get information about the sources in your image, you want different settings for focusing, for guiding, for getting data about galaxies, or for astrometry. In general the goal would be to maximize the sources detected and get reliable magnitudes, flux, etc. For solving, you don't care so much about all that, you just want to keep the stars in the image that will speed up the solver and get an accurate position. You don't even care that the magnitudes are accurate, so long as you can sort the stars and get rid of the ones you don't need.
For solving images, the profiles that I have been honing for that purpose would be the following:
FastSolving - never does multiple threads, but optimized for solving otherwise
ParallelSolve - Will do multiple threads automatically when appropriate, may parallelize on different scales or depths, or not at all as needed.
ParallelSmallScale - Just like the one above, but limited to image widths of 0.1 to 10 degrees to speed it up even more.
ParallelLargeScale - Just like the two above, but limited to image widths of 1 to 180 degrees to speed it up
You should NOT use the default options for solving.
I had a go with your images, and while some of them solved very well with the appropriate profiles, as you pointed out, there were some issues. For the file that would not open, I found that it was the alpha character in the filename. I will have to take a look at this to see why it doesn't open. I am mostly using the KStars image opening, stretching, and displaying algorithms, since this library is intended to go back into KStars first and foremost, so if there is a problem here, it probably exists there too. I will check it out. The M15 image would not solve with my profiles, I will look into that, I'm not sure why yet. The NGC 2359 image sometimes was causing a crash when opening. I think the crash is related to the KStars stretch algorithm recently developed by Hy Murveit. I will check with him to see why it might crash. But when it actually did open, it solved well just like the others.
So after I changed the alpha character in the one filename and ran all the images with the internal solver with the profiles I have developed for solving, they all solved in less than 10 seconds with. the exception of M15. I will keep playing with that image.