Well, no messages in the log that helped ... I had already tried to write to a filehandle in the directory in question, without trouble, so the directory permissions were correct.

In the end I rewrote the script that handled the uploading. The original, which worked fine on XP and IIS 6 used CGI::FormBuilder for validation and field generation. This somehow malfunctioned on the target setup, while a good ole-fashioned backend using only CGI worked. The CGI::FormBuilder version only handled file uploads up to (what seems to me to be) an arbitrary header length. In my case this meant that files under 500 bytes were uploaded, while bigger files were blocked and the script timed out. The CGI version happily handled a 40 MB upload with the same header setup. Strange.

This may be an issue with CGI::FormBuilder 2.14. I haven't tried updating to the latest version, but maybe this will help. As it is today though, I use CGI::FormBuilder for constructing the front-end with javascript validation, while a basic CGI backend handles the file upload.

Thanks for your suggestions, though :)