Instead of submitting properly the request form displays the error "Invalid Session ID." This is due to an invalid password and cached (or saved) web pages in the Chrome browser, often on Mac computers.
To Fix Invalid Session ID:
Go to Chrome Settings, expand list for Advanced Settings
Select 'Manage Passwords'
Search or scroll through the list for "ill.middlebury.edu"
Delete the password and close the browser COMPLETELY.
Try resubmitting your ILL request in a NEW session.