I fixed the problem mentioned in my initial post for one category by selecting 'portfolio' as template in the categorysettings (for 'foto & video). Then I copied this category, edited it into 'optredens', moved my two test items there, but the problem persists in that category! Beats me how that's possible. So now I have one category looking like I want it to, and another doing it's own thing

. Please help me out here.
Check two screenshots in attachment.