The spot in the center could well be a leaking shutter, I had an Ensign that had the same problem, not on every frame, but on some, turned out that the shutter sometimes was not closing fully, a cla sorted the problem out, with ilford/kodak/foma film, the main ones I have experiance with in folders, the red window would not cause this, it would need to be very bright indeed to fog the film though the backing paper, I have never seen it happen, and I rarely close the red window sheild on any of my extensive collection of folders when using the.red window, to check open the camera and hold it up to the light and if you can see a light from the shutter when closed after a few firings then you know what the problem is
Richard