@Marijn > I assume you've tried swapping cables and swapping CDJs to make sure there's nothing wrong with the inputs as well as whatever speakers you are using...?
Please take a look for the closest authorised service centre to you: http://www.pioneer.eu/uk/content/support/support/service.html
I would recommend you contact technical support - here's the contact number:
Tel: +44 (0)1753 789 789
Service: +44 (0)330 123 1240 | +44 0870 600 1539