It’s happened to a bunch of my sites over the past 3 days – some of them are sendng repeated emails about it.
Here is an example error message, in case it’s helpful:
An error of type E_ERROR was caused in line 72 of the file /home/artistworkspace/public_html/wp-content/plugins/wordpress-seo/migrations/20180515101200_WpYoastIndexableMeta.php. Error message: Uncaught Error: Class ‘Yoast\WP\Free\Yoast_Model’ not found in /home/artistworkspace/public_html/wp-content/plugins/wordpress-seo/migrations/20180515101200_WpYoastIndexableMeta.php:72
Stack trace:
#0 /home/artistworkspace/public_html/wp-content/plugins/wordpress-seo/migrations/20180515101200_WpYoastIndexableMeta.php(20): WpYoastIndexableMeta->get_table_name()
#1 /home/artistworkspace/public_html/wp-content/plugins/wordpress-seo/vendor_prefixed/ruckusing/lib/Task/Db/Migrate.php(223): WpYoastIndexableMeta->up()
#2 /home/artistworkspace/public_html/wp-content/plugins/wordpress-seo/vendor_prefixed/ruckusing/lib/Task/Db/Migrate.php(196): YoastSEO_Vendor\Task_Db_Migrate->run_migrations(Array, ‘up’, NULL)
#3 /home/artistworkspace/public_html/wp-content/plugins/wordpress-seo/vendor_prefixed/ruckusing/lib/Task/Db/Migrate.php(114): YoastSEO_Vendor\Task_Db_Migrate->prepare_to_migrate(NULL, ‘up’)
#4 /home/artistworkspace/public_html/wp-content/plugins/wordpress-seo/vendor_prefixed/ruckusing/lib/Ruckusing/Task/Manager.php