Lighttpd

Hi

I'm not sure whether this is the right place to ask as I can't figure out if this is a perl/cgi error or a lighty error.
I have an upload form that sends a file to the server. The CGI script takes the file and encrypts it. This is done in an upload_hook which is called for every 4kb uploaded data to prevent an unencrypted copy of the file on the server (as far as I understood, it should work like that...). However, if I try to upload a file larger than 4kb I get an error:

Software error:

Malformed multipart POST: data truncated

My questions are: Is this a lighty or a CGI error and what can I do to prevent it?

Thanks for your help

Best regards
Fabian Gut

Attachments:
http://paste.lighttpd.net/1012 upload form
http://paste.lighttpd.net/1013 upload script
http://paste.lighttpd.net/1014 lighty config

My System:
lighty 1.4.22 on Ubuntu 9.10