PHP sessions expire after X minutes

Since your sess_save_path is set to NULL in your config, the sessions are being stored in the path defined by php settings (session.save_path).

You may consider changing this to a path you control and have access to.
There may be some server process cleaning this folder periodically.

I would set sess_expiration and sess_time_to_update to 0 for testing purposes, and change the sess_cookie_name to something else.

Also, check your cookie settings as these may also affect session behavior.