Thick black lines on washing

[Jude]

Siemens washer dryer delivered in October 2018. Thick black lines on washing when on white wash at 40 degrees or 60 degrees. Engineer report states overloadingor wrong washing detergent. Second Siemens machine with same problem. Won't admit it is a fault with the machine. Is it because it's a washer dryer? Has anyone experienced this problem when using a washing machine which is not a washer dryer?

[Whitegoodshelp (Andy)]

Washer dryers tend to be more prone to it because of the way the heating vent for the dryer is built into the top of the door seal. Laundry can get dragged around the door seal if it gets caught in the gap between the seal and the drum. It can pick up marks from the top where the heating vent goes onto the door seal so it can pump hot air into the drum. 

Have a a close look up there to see if there is gunge or black mould. Try rubbing a white cloth around there to see if you can pick up any marks to show it’s coming from there. Also peel back the door seal in front of the drum and inspect all around for gunge.