Plesk: Upstream Sent to Big Headers

This Help Center article explains why this error message appears and how you can easily fix it.

Cause

The error occurs when a request cannot be processed by the proxy server because the buffer size is insufficient.

To resolve the error yourself, navigate to your Plesk Control Panel.

The following Help Center article explains how to log in to your Plesk Control Panel: Logging in to the Plesk Control Panel

If necessary, you can increase the values of the parameters mentioned even further.

If you’re using Nginx as a PHP handler with proxy mode disabled, navigate to example.com (your domain) → Hosting & DNS → Apache & nginx.

There, under Additional Nginx Directives, you can define the following directive:

fastcgi_buffers 8 16k;
fastcgi_buffer_size 32k;

If you are using Nginx with proxy mode enabled, you can use the following directive:

proxy_buffers 8 16k;
proxy_buffer_size 32k;

To fix the error for all domains, log in to your server as the root user via SSH.

Open the Nginx configuration with the following command:

nano /etc/nginx/nginx.conf

Adjust the configuration in the http { block as follows:

proxy_buffers 8 16k;
proxy_buffer_size 32k;
fastcgi_buffers 8 16k;
fastcgi_buffer_size 32k;

If necessary, you can increase the values of the parameters listed above even further.

After successfully testing the configuration with nginx -t, reload the Nginx service by running the following command:

systemctl reload nginx