Hi Rolf,
I think on the second site the problem is that jQuery 1.5 is being loaded. That version is too old – the current one with WordPress is 1.8.x. Here’s the line:
<scripttype='text/javascript'src='http://ajax.googleapis.com/ajax/libs/jquery/1.5.2/jquery.min.js?ver=3.4.2'></script>
Can you try disabling your other plugins / theme to find where that line is being loaded? Or you can change it to “…/jquery/1.8.3…” for the current version.