I know you gave us the admin but as already told you that the problem is related at file level , this is not at all related to theme or any of its plugin's code .
There is a php file which is fetched in the popup for the contents .
I looked into and find your .htaccess having 100s of rules in it in one of which you are redirecting deliberately from accessing the wp-content/plugins folder which is why the popup content is being redirecting .
This is the line I guess :
RewriteRule ^wp-content/plugins/.*.(?:php[1-7]?|pht|phtml?|phps).?$ - [NC,F]
http://prntscr.com/q3rtmz
I had to put my 3 hours in order to find this out which was your duty in first place to check as the nature of the issue was server related .
Please double check your .htaccess files for such rules for security and then check the popup .