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Hey man, thanks!
Because of your remark I started re-thinking the blocking. Since I did see pop-ups being blocked elsewhere I thought this couldn't be the issue.
Since the problem occurs in both Mozilla and internet explorer it had to be something outside the browsers. First I thought of XP's firewall. But that was switched of ... because I have Norton on my system! Tatata! There the solution was.
A few days ago I had to simulate the situation of a customer who didn't see the ads on our website. It appeared that he had the privacy filter of Norton switched on. I forgot to put it back.
Now it works as it did before, this feature I use so often!
Thanks again!