You can show the error messages for your page by adding "&errors" or "&errorsall" to the URL string in the address field of your browser.
Tell us which error messages come up then (you must be logged in as admin to make that work).
You can check the session settings of your PHP installation by calling the phpinfo() function. Create a file named phpinfo.php with the lines
and upload this to your server. Call the file and you'll see your PHP information.
Usually the empty session path is a server related problem. Contact your hoster and ask him about session condiguration and session path