Getting CloudFlare bad gateway error…losing sales….how to fix

Just noticed that this error is randomly coming for some products. So not all products has this issue.

Further investigation revealed that only the last product that I entered a couple of days ago is working. Tried 9, 10 random products but they all show error when I add them in cart.

I am seeing following entry in nginx log file.

) AND fs_posts.post_type = ‘size-chart’ AND ((fs_posts.post_status = ‘publish’)) GROUP BY fs_posts.ID ORDER BY fs_posts.post_date DESC LIMIT 0, 1 made by require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), include(‘/plugins/woocommerce/templates/single-product.php’), wc_get_template_part, load_

2020/09/01 03:58:34 [error] 22156#22156: *419 upstream sent too big header while reading response header from upstream, client: xxx.xxx.xxx.xxx, server: https://www.mysite.com, request: “POST /product/clothing/my-product/ HTTP/1.1”, upstream: “fastcgi://unix:/var/run/php/php7.3-fpm.sock:”, host: “www.mysite.com”, referrer: “https://www.mysite.com/product/clothing/my-product/?doing_wp_cron=1598947106.0789239406585693359375”

• This reply was modified 4 years, 3 months ago by TechMan.

Hello there,
This kind of problem is usually caused by either a conflict with your theme or with another plugin.
?
The best way to determine this is to:

• Temporarily switch your theme to Storefront
• Disable all plugins except for WooCommerce
• Repeat the action that is causing the problem

If you’re not seeing the same problem after completing the conflict test, then you know the problem was with the plugins and/or theme you deactivated. To figure out which plugin is causing the problem, reactivate your other plugins one by one, testing after each, until you find the one causing conflict. You can find a more detailed explanation on how to do a conflict test here.

I also recommend contacting CloudFlare for further investigation!

Added following lines in nginx.conf fixed the issue

fastcgi_buffers 16 16k;
fastcgi_buffer_size 32k;