I can't say anything to Nebulosity as I use the DeepSkyStacker for stacking and mainly Photoshop afterwards. Usually I equalize and normalize the white point in lightroom in each frame bevor stacking. Often the sky color changes during the the session and DeepSkyStacker sometimes fails if I do not help it. I leave all other settings in LR unchanged and set the already preset sharpening to 0 before I export for stacking. All testing with pre-processing in LR did not result in better stacking results - most were worse.

The big flare I mentioned becomes visible only with maximum stretching in the end of the processing. I can't access your Dropbox from here. If the blue circle is not present in your results you have not stretched it to the maximum extent. I think you should also see it when you apply a maximum contrast in LR by shifting the brighness ruler to the right and then the black one to the left in several consecutive steps.

You conclude in the earlier post above that the vignetting could be an effect of the old lens. All lenses have some amount of vignetting. The 200/4 is not much worse than others. But in AP when stretching the dark part of the histogram to separate background from nebulosity the vignetting effect is multiplied and becomes limiting factor for the processing. I would recommend to use flat frames to correct the image. As you use lenses and not a telescope it should be sufficient to produce and use one master flat for each lens. You can use them in every stacking process to get rid of the vignettes. You can use the "T-Shirt-Method" with two stretched white shirts over the lens that points to the cloudy sky to get the flat data. Using flats is an essential standard in AP. If there remains still some vignetting it can be removed together with other (color) gradients in the beginning of the post processing.

Sorry, I'm a little confused. You say you eq, norm, etc. in LR before stacking but then you said all testing with pre-processing did not give better results. But you do pre-processing in general or that was a test?

I tried some more stretching in LR post but can't really see the flair anymore. I think its from the synthetic flattening in Nebulosity - works pretty well but isn't perfect. Anyway, I'll keep an eye on it.

I had flat frames but when I captured, I used pk_tether and the two frame sizes don't match so couldn't use the flats! Its always something! But you make a good case for flats because I was debating about using them and normalization and was ready to ditch both of them. Now that I think of it, maybe I can crop them to match. I'll give that a try.

Just a comment on focusing - try to get in the habit of re-focusing regularly. Focus will drift with time during a session due to either thermal effects or unintentional knocking about. Nothing is as demoralizing as realising almost a whole session needs to be scrapped due to poor focus.

Sorry I'm not a native english speaker and sometimes have a lack of precison or use the wrong wording. I meant that I do an adjustment on the white balance in Lightroom before stacking. That is what I meant with equalization. So for stacking I normally use lights that are only pre-processed concerning the white balance. I experimented with pre-noisereduction, pre-sharpening, pre-stretching etc. in LR but it did not bring out a positive effect. Before export from LR and stacking I also take back the sharpening because LR applies a bit of sharpening automatically in my imports.

...or go out with a white t-shirt and camera on a grey day and shoot some flats against the sky with pk-tether to produce a pk-tether master flat.