pictures not generated, any hints for debugging?

Hi,

I installed imagecache and imagecache_profile and created two standard presets from the readme. But If the sample image won't be generated. I have no idea where to look for error messages, the directory is writeable for www-data and it doesn't work with imagemagick *and* gdlib. Even with no filters...no directory for the filter is generated and if I do that manually, the image isn't saved there anyway.

What went wrong, and how do I debug this?