This is the first scanner I have bought and I started by scanning some 120 colour negs from about 10-15 years ago which came out fine. I then tried some 35mm B&W negs from the mid 80s. I switched the little palm tree icon from colour to B&W (16 bit -> 8 bit) and the images came up fine on the screen in Silverfast. I scanned them and - everything seemed ok until I viewed them in the location they were saved to. They came out as below and I've no idea why. I tried several times and it made no difference.
I use Epsonscan not Silverfast. But have you checked the input film setting to verify it's set for 35mm? Also, is the cropping set to automatic or do you frame the negative manually? Also, make sure the film holder is not blocking the sensing inputs near the top of the holder which determines the type of film.