DP2M: Dark spot in sun rise photos

I made some sun rise photos in the mountain in Austria and in all photos I see a small black spot. The spot is only seen if the photo's are done in vertical orientation. Have a look into the example. I am using Camera JPEG's and not RAW. If there is a problem with processing than this exists in the camera JPEG engine. Does anybody else have this issue ?

I must say beside that it is the best Sigma DP camera I have ever had. The Autofocus is much faster and much more accurate than on the older models.

cheers,
clemens