Content Monitor - fatal error

When attempting to create a "new page", regardless of content placed in page, after I hit Publish I get the following:

Fatal error: Uncaught Error: Call to undefined function mb_stripos() in /usr/share/nginx/site.tld/wp-content/plugins/content-monitor/content-monitor.php:103 Stack trace: #0 /usr/share/nginx/site.tld/wp-includes/class-wp-hook.php(300): Content_Monitor->post_monitor(28, Object(WP_Post)) #1 /usr/share/nginx/site.tld/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters(NULL, Array) #2 /usr/share/nginx/site.tld/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #3 /usr/share/nginx/site.tld/wp-includes/post.php(3505): do_action('save_post', 28, Object(WP_Post), true) #4 /usr/share/nginx/site.tld/wp-includes/post.php(3578): wp_insert_post(Array, false) #5 /usr/share/nginx/site.tld/wp-admin/includes/post.php(378): wp_update_post(Array) #6 /usr/share/nginx/site.tld/wp-admin/post.php(193): edit_post() #7 {main} thrown in /usr/share/nginx/site.tld/wp-content/plugins/content-monitor/content-monitor.php on line 103

If I go back to the add page interface, the page does indeed get created.

Disabled content monitor on the site, until this can be looked into.