Thanks for your feedback. Looks like the PHP session has expired. Do you remember how many time you spend on that run? Ive implemented a method to "ping" the server every 5 minutes to have the PHP session alive all the time. This trick seems to work but after XX minutes the webserver still deletes old PHP sessions.
I think I'll have to remove the captcha and sessions, even if that leaves the application more vulnerable to script-kiddies attacks.
I will work on a quick fix. Sorry for the inconvenience.