@janvankuijk. Thanks for using my plugin. I see the error 500 when trying to upload. As for how to fix it, I can’t say whether I can help you without you providing more information. For example, I may need to know your Joomla version, PHP version, and either access to your php configuration (phpinfo). If you want to, you can send me the information via email.
Now, remember that Easy Flash Uploader actually uses SWFUpload and a search on Google will reveal some interesting information which, at least, initially leads me to believe that it is an issue of Flash 10 being incompatible with mod_security. However, at this point, I can’t be certain.
Hope this helps some.