[ThorstenA]

I tried to delete all characters that are not a-z A-Z 0-9, as it seems it does not work as I wanted it to work. The forum title is cleaned this way:

$adtitle = $prependchars . "$forum[title_clean]"; // $prependchars are the spaces
$adtitle = str_replace(" ","_",$adtitle); // spaces " "are rewritten as "_"
$adtitle = preg_replace("/[^a-z0-9\\-\\_]/i", "", $adtitle); // everything is deleted instead of a-z / A-Z characters and numbers.
$adtitle = substr($adtitle,0,40);  // limit title length to a maximum of 40.

The red line is the regexp (regular expression) code which fails in your situation. Unfortunately I am no regexp expert and it would take hours if not days to work out your bug. Maybe some more experienced programmer could take a short look into that situation? I'd then take the new regular expression and publish an updated version of this product here.