I have now come to the stage where I let the computer stay on for ages with the pump running. However from what I can tell the pump does not start.
The red LED blinks continously. I followed the troubleshooting step in the manual, and did a factory reset. The jumper is not present during launch. To reduce the chance of any issues I have removed all other cables but power. What is the next thing to try? What is the pump trying to signalise?

I have the one on the picture; yellow backplate with molex, pins, etc.

Manual:

Quoted

Initial operation
The red status LED on the rear side of the pump should now lighten up for approximately three seconds and then be turned off again. (The LED might also be blinking, depending on controller configuration.) If the LED is permanently switched on, the pump controller has detected a malfunction. In this case, please perform a reset as described in chapter 9.1.

9.1 describes the hard reset that I tried with the jumper on deaeration during power-on.

No. I have not actually timed it, but this is how the duty cycle appears:
[on - 0.7 sec] [off - 0.3 sec]

Everytime the light goes off there is a very very soft jolt, as if the electric engine tries to go over 2-3 "peaks" (as is common with electric engines). But it does not commence continuous operation.
The deaeration pin is not present. However I suppose this is how deaeration is supposed to work? Maybe it's stuck in deaeration mode despite the hard-reset?
I have access to a second computer, and thus could possibly access the pump via the software. Could this provide any means to fix it?
Edit:
I have now tried to connect it to USB to no avail. With vista 64bit Windows comes up with the following message: The device has malfunctioned/Device not recognized