I just inspected this in the frontend:
When I click the button, an XHR is started and replied with code 200 OK
It seems a whole page is returned including <head> and page header and footer menu bars.
I wouldn't expect this from an XHR but maybe it was implemented that way.
However, the body contains the following suspicious lines:
<div id="fatal_error">
<div class="cat_bar">
<h3 class="catbg">
An Error Has Occurred!
</h3>
</div>
<div class="windowbg">
<span class="topslice"><span></span></span>
<div class="padding">Function name must be a string</div>
<span class="botslice"><span></span></span>
</div>
</div>
Maybe searchcing for that "Function name must be a string" string helps.