I can't upload any image anymore

this is the error:
image.jpg 33.42 KB Fout: true

if J upload via ftp software, it works

Please check your logs as always. It can be lots of things. FTP of course always works as here neither Zenphoto nor any other scripts are involved.

it was a permissions problem, I can upload now, put when I click on a link on the site www.michel-nulens.be there is a 404 error
All folders are set to 755 and the files to 644

Just ttried the site and all works for me. Except that you are not using modrewrite.