Just thanking people (especially Marc) for the contributions to this old thread. Helped me with the same issue today which I've had a few times recently but only now had the time to debug.
I was getting strongly over-corrected images using PixInsight (probably ImagesPlus also if I recall the last time I used it). Adding Bias frames and reducing the brightness of the flat has much improved the result. Not actually sure which of those two did the trick but I'm a happy chappy now.
DBE in PixInsight is now freed up to fix the remaining issues more easily
.
Phil