Access Is Denied to Server Share from Forms Auth Web App

Hi,
I have a solution with several projects in it. I want to provide access to a shared directory on the server, but I keep getting the error "Access is denied". The relevant app pool is running under NETWORK SERVICE and I have set permissions at the share level, and at the NTFS level to allow access, but to no avail. It's OK in all the other projects in the solution, (which use Windows authentication and the same app pool), but not this project which uses forms auth. The other thing is that the site uses an https secure connection, though don't know if that has any effect.
I can access the directory by directly typing the "file://server/file" link into the browser address bar, but if I click on a link on the page with the same address, no dice. The same situation happens if I try to open a window server-side with "Process.Start".
Event code: 3005 Event message: An unhandled exception ...