Add Media button does not work

When Trying to add an image using the button, nothing happens (lietrarly). This worked before, but does not anymore. When I add images to the library I also get an empty white page, but the images are uplaoded because I can see them via the theme editor (not the media library).

This WP is on the company system and they do not provide any support, so I’m hoping someone here has an idea how to fix this.

I already tried:
– Chrome/IE/Firefox
– Turning off all plugins
– Chosing another theme

Looking al the console (f12) gives the following error directly when opening the edit page (not even clicking on anything);

manageInappropriateContent.js?ver=4.1.1:469 Uncaught TypeError: Cannot read property 'slice' of undefined(anonymous function) @ manageInappropriateContent.js?ver=4.1.1:469j @ jquery.js?ver=1.11.1:2k.fireWith @ jquery.js?ver=1.11.1:2m.extend.ready @ jquery.js?ver=1.11.1:2J @ jquery.js?ver=1.11.1:2
bridge.js:1 Connecting to PORT_NAME_SYMANTEC_PKI_CLIENT_CONTENT_TO_BACKGROUND...