As reported here and solved here. Briefly: "Thread prefixes" unnecessarily declares $config as global in tpref_info(), and because $mybb->config is set to $config by reference in inc/init.php, when a string is assigned to $config in tpref_info(), $mybb->config becomes inaccessible as the array it is supposed to be by Google SEO in google_seo_plugin_status(), leading to a fatal error and the white screen of death on the plugin page.
As reported here and solved here. Briefly: "Thread prefixes" unnecessarily declares
$configas global intpref_info(), and because$mybb->configis set to$configby reference ininc/init.php, when a string is assigned to$configintpref_info(),$mybb->configbecomes inaccessible as the array it is supposed to be by Google SEO ingoogle_seo_plugin_status(), leading to a fatal error and the white screen of death on the plugin page.