We’ve found the problem.
Our latest plugin update, 3.6.4, will not work properly with sites on PHP 5.3.* or older. It requires PHP 5.4 or higher to function properly. 99.9% of our users meet this PHP requirement, but a few users are on PHP 5.3.* or older and are encountering problems.
If your site runs on the older PHP 5.3.* you can still use our previous plugin version, v3.6.3, without a problem. It can be downloaded directly from WordPress at: https://wordpress.org/plugins/wordpress-23-related-posts-plugin/advanced/
We apologize for the inconvenience this has caused, and want to thank you all for helping us in discovering and diagnosing the problem.
Today we will update our readme to list PHP 5.4 or newer as a requirement for our future plugin updates. Additionally, we will build in functionality for the plugin that checks PHP version on activation and only allows activation to complete if the minimum PHP version is running on the site (if an older PHP version is detected, it will display a message directing users to our plugin version 3.6.3).
Thank you for your understanding and help.