Hi Jurgen,
Eventually something must happen. Does the report finally finish? Do you quit and close your session? I would take a look at administration overview tab and see if you see memory consumption growing rapidly and also the performance tab to see what exactly is running. Is this a large table perhaps and you are running into memory issues? ie: when this happened to me in the past I would eventually get an OUT OF MEMORY error for example. As it was happening I would monitor memory and see it gradually being consumed.
-Patrick