Yes, I know, and already changed that. But fortunately something else did the trick for me:
In the configuration.php file on rule 31 and 95 I added a forward slash before the webassist url.
newImg.src = ((settings.dds && settings.dds != "")?settings.dds:"") + '/webassist/kfm/themes/webassist_v2/icons/select_none.png';
var kfm_url="/webassist/kfm/?theme=webassist_v2";