I received a 502 error on my website and the log says “Upstream sent too big header”

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.

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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