Upload don't work if running as NT-Service

I'm running antclick as a NT-Service. Installed via Helmaservice.

But now every time the upload fails (images and files). I've got only a error message and in the input field I typed the filename in the string "helma.util.MimePart@118278a" appears (using Opera browser, using IE I see nothing only the error message). In the log files I can't find any error messages.

I assume it could be something to do with filesystem permissions but all relevant dirs have full access rights.

If I don't run antclick as a service all works well. What's wrong?