Widgets setup not loading correctly

Hi,

When I go to Appearance\Widgets, the page is not loaded correctly.
When looking at the source code, you can see it stops loading halfway on, the following tag:

<div class="widget-inside">
<form action="" method="post">
<div class="widget-content">

So the available widgets are not shown in the “available widgets box”, but beneath it.
The sidebar to drag the widgets to is also not loaded.
I get no error message, also not in de IIS logs.