Site breaks after plugin activation

Whether installing via the directory or manually by upload via git, once Custom Sidebars (Version 2.1.0.2) is activated, our whole site goes down, with just this appearing where pages used to be:

Warning: session_start(): user session functions not defined in /srv/bindings/403ceb4965c5417d8e20fea83b05de70/code/wp-content/plugins/custom-sidebars/inc/external/wpmu-lib/functions-wpmulib.php on line 69 Fatal error: session_start(): Failed to initialize storage module: user (path: ) in /srv/bindings/403ceb4965c5417d8e20fea83b05de70/code/wp-content/plugins/custom-sidebars/inc/external/wpmu-lib/functions-wpmulib.php on line 69

Had to delete Custom Sidebars plugin to get it to deactivate. Any ideas what could be causing this?

Current site address (Custom S: http://dev-image-journal.pantheon.io/

https://ww.wp.xz.cn/plugins/custom-sidebars/