Hey Peter, thanks for your interest! To be honest, I could not be more happy, your plugin is working perfectly and is saving tons of server resources.
I believe to have found the cause of the issue (at least for me). When I first started with WP performance optimisation, I was using Ubuntu servers with Varnish and Ryan's Memcached Object Cache plugin. Then I moved to CentOS and started experimenting with some alternatives but forgot to remove the object-cache.php file from the wp-content folder of my websites. When I moved the sites to new servers, I did it using a shell script which got rid of all remaining object-cache.php files. Since then, I had not a single issue and performance is far superior than it was before.
I don't use Varnish for WordPress sites anymore, but is worth to mention that beyond the NGINX / Memcached cache system, I also have CloudFlare Pro in front of all my websites which guarantees that all my assets are cached in a load balanced CDN.
I'm still looking to have Memcached listen on a socket instead of a TCP port. In fact, this is quite simple to achieve, however, the plugin don't work with this configuration and I could invest the time to investigate the reason deeply enough. Some days ago, however, a member have succeed in making Ryan's Memcached Object Cache plugin work with a Memcached instance listening on sockets, if you can have a look, it would be awesome.
Again, thanks a lot for making my websites super-fast!