Error with sidebars pro on Pantheon

Trying to move a site to the Pantheon Servers and I got the following error.

Warning: session_start(): user session functions not defined in /srv/bindings/75560645ba3c49eaac68f08c8b9f0b8c/code/wp-content/plugins/custom-sidebars/inc/external/wpmu-lib/inc/class-thelib.php on line 114 Fatal error: session_start(): Failed to initialize storage module: user (path: ) in /srv/bindings/75560645ba3c49eaac68f08c8b9f0b8c/code/wp-content/plugins/custom-sidebars/inc/external/wpmu-lib/inc/class-thelib.php on line 114

Please help