I dismounted the rotary pump (Fluid-O-Tech, CB054Z), I opened the balanced bypass nut (20), got in my hands the balanced bypass screw (22), bypass plunger (19) and spring (14). My pump does not have the lock nut (21).
I found nothing wrong, apart from the bypass plunger which is a little bit hard to move and the bypass nut has been turned out by me that much that the spring has no function (does not push) any more.
I am still where I was before dismounting the pump: much more water comes out of the expansion valve drain than arrives in the cup, in whatever setting of the bypass valve or the OPV. My best guess for what's going wrong: the firmness of the bypass plunger. I do not know whether this is the actual reason, nor whether I can simply put some grease on it.
I agree with Eric: "... avoid, if at all possible, sending the machine to a service center ...", but I want to arrive at a solid solution. One option (it might be exaggerated, but it is pretty affordable, about US $100) is to simply replace the Fluid-O-Tech pump. I found it pretty hard to turn my pump by hand, turning the pump with a screwdriver is easy. Turning the pump motor by hand is really a breeze.
Suggestions welcome, Leendert.