404 error on body of page (header + sidebar works)

I get a 404 error and cant figure out why. The header works, the sidebar works and the posts work. I only get a 404 on the content of the pages. I reset the permalinks to default, nothing happens. I deactivated all the pluging, no result, I manually changed the .htaccess, no result. I have a test.php and test.html that can be reached. I changed the theme back to TwentyEleven .. now I can see my pages again. which leads me to think it could be something with the theme Catch Evolution (I'm using a child theme). The site is: http://www.gzpsycholooggroningen.nl/ Any tips are more than welcome!