We’ve seen this error with the WordPress plugin OptInMonster, though it certainly could pop up in other circumstances too. The issue occurs when a PHP script responds with more header data than nginx (the front-end web server) is able to accept.
The solution is to add the following lines to your nginx configuration in Plesk – note that if you’re on shared hosting you’ll need to open a support ticket to have us do it for you.
fastcgi_buffers 128 4096k; fastcgi_buffer_size 4096k;
This tells nginx that it’s okay for it to accept ridiculously large (4MB!!) headers. Click OK or Apply to save, and try again – you should find it no longer has an issue.