AJAX error when adding a selection rule

Hi, I'm new to Drupal, and at this moment I'm trying tutorials to get to know the modules.

However, I get the same error on my 2 'sandbox-websites'. One of them was a fresh one to get to know page manager.

I didn't have any other problems with Page Manager or AJAX so far.

Issue:

Whenever I try to add a selection rule to a variant in Page Manager, I get this error:

An AJAX HTTP request terminated abnormally.
Debugging information follows.
Path: http://www.paridaen.be/sandbox/ctools/context/ajax/access/add/page_manag...
StatusText:
ResponseText:
ReadyState: 4

What I've tried

I used different browsers (Chrome, Safari, Firefox, Opera)
I deactivated the other modules, but the error still occurred.
I've tried chaos tool suite 7.x 1.1 and 7.x 1.2

Thank you for your time!