Thanks for the files. Both images are 120 seconds exposed and solve without problems and enough detection's. Brightness doesn't disturb. Even the center star is used for the solving. Hot pixels are ignored See detection's below. The limiting magnitudes are 15.6 and 16.3. Then there are enough stars detected. I also looked to some Iris nebula images I could retrieve from nova.astrometry.net and cropped them to about 0.55x0.37 degrees. Some did solve some not. The one which solve did go deeper and showed detectable stars up to magnitude 17.3.
So since it is a little dark corner of the sky (for 0.55x0.37 degrees), the images should go to at least up to magnitude 15.6. For this the images should be reasonable in focus and exposure time should be long enough.
In general nova.astrometry.net requires less stars to solve. ASTAP will require about 30 stars.
So it is unclear why in previously it didn't work for exposure up to 240 seconds. So if possible I would suggest to try next time again and save some images of shorter exposure. If your image goes deep enough it should work.
Below the detection result of the XISF file up to magnitude 16.3. Red squares are star detection's. Yellow markers are stars used for the solution. You can generate this image by check-mark "Show extended log" in tab alignment.
Ye I am waiting for a chance with the weather. It looks like I have another week of rain coming up.
The position of the images I sent you would plate solve at 15 sec exposure with H17 with no problem. It would be the next plate solve to the center of the nebula that would fail for both NINA and APT using ASTAP (Thus my thought about brightness "blinding" ASTAP). Those were part of a session where the star was close enough to the center of the image so I ignore the final centering and went on with it.
I think that is why Pixinsight PCC plate solve fails and if I change the fit header with the right center coordinates then it solves. Just a speculation as I do not know much about this.
I know that about a year ago or so, I would have to lower the exposure very much for plate solve otherwise it would fail. I was using APT in case that matters. IIRC that was discussed around the forums and fixed.
I have also seen this happen when trying to center bright stars like Sirius or Vega with my AT6RC at .36 arcsec/pix. But that was a while ago and I do not remember the exact conditions.
I will send you images when the weather let get some.
Thank you for your explanations.