Thanks for your quick reply Muhammad. it is well appreciated.
Here are the versions of the plugins I am running.
Yoast WordPress SEO Version 2.2.1
Random Featured Image Version 1.1.2
Note that this is only happening on 5 of my sites. I have 30 other sites that all have the same lists of active plugins, and Yoast WordPress SEO and
Random Featured Image work fine.
So I deleted Random Featured Image, did a refresh, then reinstalled Random Featured Image. During the reinstall I got error message “The plugin does not have a valid header.”