I forgot to mention that I did check some things told in the help file, but I couldn't find the specific htaccess file and where to place it when you use Joomla.
I found the following code in my server log:
Code:
[Thu Oct 16 19:53:10 2008] [error] mod_security: Access denied with code 500. Error processing request body: Multipart: final boundary missing [hostname "mysite"] [uri "/administrator/components/com_joomla_flash_uploader/tfu/tfu_upload.php?dir=&remaining=1&TFUSESSID=uma1jat08afg8s4lv073vec6t1&size=10000&firstStart=true&zeit=1224179575214&tfu_rn=1227288324934484294314173091658"]