ngx_slab_alloc() failed: no memory in SSL session shared cache “le_nginx_SSL”

The other day, I setup an Nginx web server on Ubuntu 14.04.5 LTS and used certbot to install a Let’s Encrypt SSL certificate. However, soon enough, a strange error began to appear in my Nginx error logs.

The error in question was:

ngx_slab_alloc() failed: no memory in SSL session shared cache “le_nginx_SSL”

Fortunately, this error isn’t fatal, as my site continued to work fine despite it. The issue seemed to stem from a configuration setting in my /etc/letsencrypt/options-ssl-nginx.conf file.

The line in question was:

To fix the issue I simply commented out the line in question.

At the top of the file, you might find the following warning message:

Keep this in mind!

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