Strange circles in the photo

Observatório Astrográfico do Boqueirão:
When I made the subs on my refractor a few weeks ago I had the same problem and my guess is related with dew or temperature in my case. I made two pair of flats and despite the flats solved as usual the vignetting and dust spots, It didn't solve that circle as you have. But in your case I think is more related with the coma corrector.

***  At first I used a flat from a month ago, but then I made a new flat and it looks identical in both cases. As you say, the vignette and the dust have been removed, but the circle remains. ***

Is this also visible on the light frames? If not, run fast integration and see what's the result without darks/flats

The luminosity of the image is very high. The circle should disappear after appropriate processing.

This may not be an issue that you're having, but I notice that the shading on the big circle is not symmetrical. On the left side, it is dark on the inside and mildly brighter on the outside. Compare that with the right side, where it is light on the inside and dark on the outside. 

I had a similar issue with my QHY268M (which is a mono cousin of your camera). The cause of the asymmetry was due to the filter wheel not returning to exactly the same spot depending on which direction the filter wheel was spinning. QHY issues a software patch to allow the filter wheel to always spin in the same direction, which fixed the issue. I've never heard of this issue with ASI cameras, but I haven't been paying close attention.

External reflections, moon, lamps etc. Anything in the vicinity?

halogen lamp in the neighbor's garden...

*** Could be that, they are quite bright

Sergey Kholmanskikh:
I experienced similar kind of things with my 6" Newton. Usually, it's scattered light from the Moon or neighboring lamps leaking into the optic path, mostly the light falling directly into the coma corrector which is missing the mirrors. This might be the case if your coma corrector is protruding far into the focuser and the focuser itself is close to the edge of a tube like it is in the Quattro Newtons. A blackened dew shield should be your remedy. Also, I used to have an annoying issue when the light reflected from snow on the ground was leaking through the opened back of the main mirror.

*** Thank you. This makes sense, because the problem always occurs when I take photos when the moon is already shining brightly. I'm currently using a dew shield on the front. I also have to think about covering the back of the tube ***

Tristan Campbell:
I'm having similar issues with my 150 quattro. Also have a neighbours light landing on the scope all night. I've completely sealed the rear end of the tube, think I've eliminated light from the focuser (Baader Steeltrack doesn't really fit a 6" tube) and have a 12" flocked dew shield installed. None of these have so far solved it so I'm not certain where the light or reflections are coming from but suspect its the coma corrector as already mentioned. Narrowband is fine, it only affects broadbands. Below is a flat with a DBE applied which shows very similar marks. I might try adding a baffle just in front of the spider in attempt to prevent stray light around the focuser. 

*** It seems that the problem is quite common. I use Sky-Watcher f4 Aplanatic Coma Corrector dedicated to f4, but as you can see, it doesn't change anything. I wonder if such problems would also occur with the Optolong l-Quad filter. Thank you for sharing this with me ***

The focuser I am currently using is the OVL Dual Speed 2" Low Profile Crayford Focuser For Newtonian Reflectors, which I closed with black tape around the telescope joint. Baader diamon steeltrack I received it a few days ago and  waiting for installation. What you did is an interesting idea. I will try to connect my ASI120 camera. I'll see what the effect will be.

Marc-Antonio Fischer:
Just want to let you know, that i got that problems for years with both of my quattros 150 and 300. Ive used every coma corrector and flatmethods. Nothing works. They work when I image outside of the city perfectly fine. It is 100% something going on with the brightness of the background.... i still need to figure it out. Abother person with that problem, said he changed the spider with a solid cnc one. That problem was gone then. It seems like a mirror shift can cause that proboem too. I just checkt mine and got a cnc spider. Did not work for me.. i am living in b6/7

***I have cnc spider. About a month ago I managed to photograph Andromeda during the new moon and the frames were defect-free. Now there was a moon of about 45%, but also strong light from the neighbors. All the photos I took with the broadband filter during the moon had this problem...***

Thank you Mark. I will try to seal the telescope more tightly to prevent unwanted light from entering. Maybe this will help a little. I agree with you. I also suspect light pollution. This topic was also discussed on cloudy night and it looks like a lot of people are struggling with it. They wrote about sealing the telescope. Someone also wrote that replacing the coma corrector  with another one helped.