Red circular banding due to flats

andrea tasselli:
I wouldn't say that those lights are unrecoverable at all. That is where DBE comes in handy, if you know how to use it.If you post the stacked up light I'm sure we can help you there. I'd suggest you take your flats without NINA help (you are not the first to have issues with NINA's flats), at dusk pointing the scope at the zenith and away from any source of light and no significant moonlight. Inncidentally, you could use one of you previous master flats, if you keep them.

Thanks for your help!

Inusable was probably an overstatement haha 
I guess DBE can correct part or most of the issue but I would prefer to fix the root cause of the issue rather than use processing techniques to make it disappear. 

What concerns me is that, without understanding the cause of this issue, it may reappear during my next imaging night. And since I have very few of them because of the weather, I don't want to waste any data  

However, if all else fails, I'll try my best to perform a DBE and ask for help if needed

Your dust donuts are subtracting okay, which suggests your overall flat exposure is okay–if it wasn't your donuts would over or under correct. Also, the luminance looks okay over your image, just the colors are not right, forming those colored rings. If you think about it, there is no way a dust donut should be biased green or red, they block light period, so should be neutral in the debayered flat if everything is okay.  So the biased donuts seem to correspond to the colored rings you seeing in the flat. 

It looks like there is a stray source of light at the center left of the flats–light is getting in somewhere, or reflecting somewhere, or non uniform in some other way, creating these concentric colored rings centered on the left center edge. They might have been some reflection from a filter, but you aren't using one. 

You have done a lot right with your flats. You need to make sure everything for your flats is the same, which you have done. Exposure is usually difficult to match, but that is okay, as long as you have appropriate darks for flats and lights, which you do.

Some further suggestions for the flats, try things one at a time to isolate the issue, even if it is is much after the fact. I have rescued images with flats taken much later, when something changed in the setup during my night observing:
1) Are you in a totally dark place when doing your flats, so that stray light cannot get in? You might even want to have something that blocks extraneous light from the iPad, with just a hole cut in it for the light that enters your lens.
2) Take a few sets of flats, with different orientations of the iPad, rotate it for different flats, and see if the center of the colored circle moves around from the left edge–the iPad area may not be that uniform in brightness across the screen (even if it was in the past). You could even take a few flats with just room light, or using the sky, see if that makes a difference and gets rid of these colored rings.
3) Your average flat exposure appears to be okay. But you need to check the histogram of all colors–typically you will find the green histogram peak  is to the right, while the blue is in the middle and the red to the lower end left.  You need to make sure no histogram peaks are too close to the top/right (usually green) or at the bottom/ left (usually red) of your histogram trace. If either of those channels are in non-linear regions, or even perhaps clipped, the different color channels will behave differently, potentially causing those colored rings.

It is a process of elimination, try different things and see what happens. 
Good luck
Rick

Yeah, I guess internal reflections are a possibility.

Just throwing this out here since I had my share of weird issues - any chance there is some dust on the chip? Depending on what this dust is made of, it might actually refract light. For me, I actually had tiny amount of metal flakes landing on my filters whenever I screwed my camera into my scope (tight threads, I guess). Which then seemed to have caused individual stars to look completely bonkers, with weird refraction patterns around them. After a careful dissassembly of the camera and some brushing/cleaning, those isses are now gone. But it was super-difficult to even see by eye. Now I always make sure I screw on the camera with the whole assembly being in a horizontal position… probably just superstition, but you know…