Yes, I meant the new uploading feature in WP admin. The file and folder permissions seems right, no problems there.
Maybe I have found the problem:
In the database, table “postmeta”, the “meta_value” (file path) doesn’t have slashes correctly written and there is some odd characters in the path. I’ve tried changing this path manually (after uploading a file) and that makes the delete function work (for that file only of course).
Maybe this is a bug in WP?
Don’t know if it makes a difference, but I’m on a Windows Srv 2003 machine, running Apache 2.0.55, PHP 5.03 and MySQL 4.1.16.