I have a problem with the projects that I develop that intermittently generates a session error, but the error is only when the project is hosted on the web server (I use CPanel from a reseller of HostGator).
When the error occurs, if the user presses "F5" or simply change the page (when possible) the error disappears, so far I have not been able to generate the error twice or more.
Follow images of the error.
Followingistheimageofthecodeused(itishighlightedintheyellowrectangle),whichisrotatedandmaypossiblybecausingtheerror. 
Site url so they can try to generate the error: www.ravsistemas.com.br NOTE: In order for me to be able to generate the error more quickly I usually open the site in two browsers and in one of the browsers I change pages or refresh the page.