;-/ Ahhh.... suddenly it all became obvious. The rotation is ... round. Didn't think of that.!
Anyone with an idea on what frequency - how many rotations per minute or second?[/quote]

You apparently have one of the "3000" series tanks. According to the JOBO instruction manual for the CPP2, using a 3000 series tank, with 4" x 5" film, the rotation should be set at "F". I've never timed the frequency, but all I ever use with either 35mm or 120 film is "P" - which, if memory serves is about 75 RPM - or would be if the rotation did not reverse itself every two(2) revolutions. "F" is the slowest rotational speed available - and I am going to guess at about 30 RPM ... If it was "one way" ... etc. - see previous.

There is one instance - with 110 film - where JOBO does NOT recommend reversing rotation - and suggests that the reversing cam be "switched out of the way" so that the drum revolves in only one direction.

Now - my opinion, for what it is worth: I have an idea that the rotation, like the agitation of a hand-held tank, is not really all that critical. I'd start at about 30 RPM, and increase the rotation speed if I detected uneven development.