From time to time blemishes at the middle of film appear as you can see on picture below. I use Paterson plastic tank and ID 11. Whatever the film is I just can't get rid of those. Any advise is most appreciated...
I'd go so far as to say that a Paterson tank with ID 11 are the "vanilla" choices we've all made at some time and they're not the cause of this problem. I'm assuming this is 35mm film, since 120 would give a different problem depending on the film ratio? If this is longitudinal in the centre down a 35mm film then you may have a camera issue rather than a processing one.
The streak doesn't look uniform along the negative. Did you perhaps hang up two rolls of film to dry and they touched, or maybe your film accidentally touched something while drying. Just a thought...
The streak across the middle looks like Newton's rings to me. They are caused by the film touching the glass of your scanner and forming concentric rings, which can sometimes be (like in this case) quite elongated. Also make sure you give your film plenty of time to dry before scanning: four hours at the very least, overnight preferably.