I can read the session.save_path from the remote server at php_info.php. It is set to "no value". I assume that this defaults to /tmp.
I understand that in the absence of some other command, this means that session variable files will be saved here.
The error message is
Warning: session_start() [function.session-start]: open(c:/wampserver/tmp/sess_prrfb9ljt01401ls8oh4un7qi5, O_RDWR) failed: No such file or directory (2) in
It's trying to open a session file using a Windows-style filepath. It might be a configuration parameter being set inside my PHP code, but a local site search of 6600 files for "wampserver" does not come up with anything.
I think this means that the live copy is trying to open a file in a path which doesn't exist, hence the error messages. I know and understand that PHP will stick to its configuration file on the web server, which will normally be the system default, but given all this I cannot explain or understand the error message looking for c:/wampserver/tmp.
What's more, if I go to my local server and amend php.ini to read session.save_path = "../../portal/woodwork_sub/sessions" for example (instead of session.save_path = "c:/wampserver/tmp"), but without creating the folder named "sessions", I then get exactly the same error message as is appearing from your server.
It seems to me that there is something wrong with the session.save_path at the remote php.ini .