Hey,
I do not know what the reason for your internal server error is, but for your uploading problem I might have a solution.
There's a free
Java API you can get from
jspsmart.com. Unfortunately this is a commercial site, so you have to register. The API as well as the documentation is free. Look out for "jspSmartUpload".
As far as I know at
sourceforge.net there's some other API for this kind of problem. I'm not sure if this is a explicit German site.
Greetings
H.-Gerd Rosarius