Scania Starting issue

Why did you have to change the starter in the first place?
If it was just an electrical issue, and not the same symptoms as you describe with the second starter, then it sounds as though the second one is not compatible with the ring gear. I’ve once had to fit a starter on which the pinion was slightly different to the proper one to keep me going for a couple of days till I could get a new one. It would start the engine no bother, but you could hear that it wasn’t right, and sometimes I’d have to start and stop the engine a few times before the starter would disengage with the flywheel.