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.

About Allen Pooley

Allen is a self professed geek and technology lover. He's always playing with one of his various websites, and loves helping customers with theirs. He can often be found with a coffee (light roast, please) in his hand and a smile on his face... or with a plate of bacon. Mmm, bacon.

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