I had a similar problem with sap web sessionid and I could solve following the next steps:
- record a new HTTP/HTML script
- enable on RTS extended log the data returned by server
- replay the script
- search within the server's returned data where is generated the session id
- save to a parameter the session id using web_reg_save_param_regexp
- replace the session id by new parameter
that works for me :)
Aleix