When editing a newsletter, I get a 500 error when creating

When editing a newsletter, I get a 500 error when creating the newsletter.

The error I see in the console is:

POST https://website.com/ 500 (Internal Server Error)
load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,json2,utils,plupload&ver=4.0.1:4 POST https://website.com/ 500 (Internal Server Error)