It is possible that the user running the web service doesn't have access to that folder (although it would be a pretty big oversight from your host).
What you can do is create your own sessions folder and override that path using the session_save_path command.
Just create a folder that is below your http root (basically not accessible from a browser, or in other words, if your website is all in "/public_html" when you're FTP'ing, create a folder like "/php_sessions"), and set the permissions to be 0777 (everyone full access). Then set that full path (you should be able to get your full root path from a phpinfo(); ) into the sessionPath.php file.
Hope this works.
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