FDF problem with IE - any experts out there?!?

Hi! Sorry for the 'longish' post... I am working on a script that generates FDF data and sends it to the browser for display in a PDF file. I started out with a really simple script that just creates the FDF (fdf_create()), sets a value (fdf_set_value()) and PDF file (fdf_set_file()). I the fetch the FDF to a string, close the FDF and echo the string. This works for both Mozilla and MSIE: The FDF gets downloaded, Acrobat Reader starts, downloads the PDF file and displays it with the contents filled into the form fields. Fine. Now I have a slightly more complicated script, gathering some data in some forms, using a session to tie those together. The collected data is handed over to fdf_set_value() calls, and the FDF is created (more or less) exactly as in the first script. But it works only in Mozilla, MSIE starts Acrobat Reader which then pops up an alert window saying (translated from german) 'An error occured while opening this file. The file does not exist.' Can anybody help me with this? Here is what I tried so far: - switched off Cookies for session id propagation - moved from POST to GET for fetching the FDF - moved the FDF generation into it's own script Nothing worked so far, and I'm getting somewhat upset slowly but steadily. So please help! Regards, Karsten