File field not working

When clicking on upload file in the general file field nothing happens (link is #). No modal file appears.

What I have done: - I have reinstalled EE to 2.10.1 and reuploaded the cp_themes folder a couple of times. But nothing works out, the modal window still does not appear. - My mirror site (2.7.3) works fine on behalf of the modal window.

What could it be? I can’t figure out. - I have installed MSM for 4 sites all together. Could it be related? - I installed the dutch language pack. Could it be that somehow that part has not been updated in relation to the 2.10.1. - Plugins installed (a.o.): Playa, Matrix, Wygwam, Super Search - Have I missed out on any preferences?

I bug my head how to solve it and my client needs the option to add files in entries.

Best regards, Harold