URL category generates error 404 page

[ledaanbieder]

After migrating to 1.5.6 the categories are redirecting to the 404-error page (also via site map). If searching for a product and selecting this, the URL is working.

The normal CMS-pages work well:
http://www.ledaanbie...tent/4-over-ons

The productpages also work well:
http://www.ledaanbie...-warm-wit-.html

Only the categorypages don't work:
http://www.ledaanbieder.nl/155-e27-led-peer
=> This generates the 404-page: http://www.ledaanbie...?controller=404

I've tried to regenerate the .htaccess, disable and enable Friendly URL's, emptying smarty cache, but nothing seems to work.

When viewing the site map page the URL's look good, only when selected the 404-error page is also generated.

Does anybody know what is going wrong?