(FIXED) Error: PHP Fatal error: Class 'WP_Ajax_Upgrader_Skin' not found PHP Fatal error: Class 'WP_Ajax_Upgrader_Skin' not found in /plugins/wpmudev-updates/includes/class-wpmudev-dashboard-upgrader.php on line 63

I saw this error in the logs when diagnosing why one Wordpress site was not being added to WPMUDev (premium) through Auto-Sync. 

And even after I installed the WPMUDev plugin, communications to the site from WPMUDev were failing.

[26-Aug-2017 19:21:30 UTC] PHP Fatal error:  Class 'WP_Ajax_Upgrader_Skin' not found in /home/phil1/public_html/_MySite.com/wp-content/plugins/wpmudev-updates/includes/class-wpmudev-dashboard-upgrader.php on line 634

 

I was able to resolve this by updating the Wordpress site from 4.7 to the latest version as of this writing which is 4.8, followin the suggestion here