Does this mean that about 250 person visited the launch page, but
only 39 of them started the experiment?
Not necessarily. It might be that search robots crawling your launch page increased the subject count. Where did you host your experiment and how did you advertise it?
2) the variable computer.ipaddress.1 is in about 50% 0.0.0.0 in my data ,
while in the other 50% it is an ordinary ip address like
192.168.178.23. Why is that?
Today's computers have multiple network interfaces (LAN, W-LAN, etc.). 0.0.0.0 indicates that the first interface (computer.ipaddress.1) on a given computer (e.g. LAN card) was not assigned an IP address. computer.ipaddress.2 might have returned something else.
3) Is it possible to track the participants referrer in the data with a
http-referer? This would allow me to monitor the success of online
akquisition strategies...
Not directly. You would have to apply much of the same strategies as in https://www.millisecond.com/support/docs/v3/html/howto/interopsurveys.htm, i.e., use query parameters.
4) I included also a stat counter on the ending page. Unfortunately it
shows, that 31 people "completed" the experiment, but in fact only 23
completed. Why is that?
Not sure, but might be related to (1). Also not sure what exactly you consider the "ending page".
Regards,
~Dave