Yep, there are several forums in which this happened and I can't explain it. There must be about 15 ore more of these strange posts in several forums.
I wrote a mail to
info@vbulletin.org now to have a look at it. Because I don't want to get a bad name for things I don't do.
Thanks anyway for the reply. Maybe with this the admin can have a better understanding about how this problem occurs.