Thanks, this is the most convincing approach so far. I just looked into this. The foot of the middle tube and the counterpart in the tank have notches thus that the spool is fixed. There are two ways to put the spool in. In one position there is a slip of about 20° in the other the slip is about 5° to 10°.Actually it looks as though the reel with the sheets of film isn't rotating properly causing the banding, could be slipping.
The tank itself is coupled to the CPE2 by a magnet. This may be another cause of slip if the rollers are causing so much friction that the tank has a slight slack as the direction of the rotation changes. This may cause irregular development square (!) to the direction of rotation. What I can not see is the cause of the very regular recurrence of the stripes.