I searched the Support Page and found this article that may help you troubleshoot this issue.
Since yesterday I get a vertical black/gray streak (about 2.5 cm) on the left side over the whole back page when I scan a two-sided document using the ADF of my WorkCentre 6515.
I already cleaned the scanner, but it didn't help. The 6515 has the latest firmware update 65.68.62.
Any ideas what's wrong here? (I only attached a photo of the ADF scan of the back side of a two-sided document; the front side is scanned without any issues.)