Plugin Installation breaks WP Instance on Pantheon Website Management

Hi,

I am getting error as below using pantheon website management tool:

Warning: session_start(): user session functions not defined in /srv/bindings/19198c0dfcdd46aa847c4e860bc78492/code/wp-content/plugins/ultimate-member/core/um-filters-members.php on line 245 Fatal error: session_start(): Failed to initialize storage module: user (path: ) in /srv/bindings/19198c0dfcdd46aa847c4e860bc78492/code/wp-content/plugins/ultimate-member/core/um-filters-members.php on line 245

Please check and let me know.

https://www.ads-software.com/plugins/ultimate-member/