The "upstream sent too big header while reading response header from upstream" error in Nginx can occur when the size of the HTTP headers in the response from the upstream server exceeds the value of the large_client_header_buffers directive.
By default, the large_client_header_buffers directive is set to 4 8k, which means that Nginx is configured to handle HTTP headers up to 8 kilobytes in size. If the upstream server sends headers that are larger than this, Nginx will return the "upstream sent too big header" error.
To fix this error, you can increase the value of the large_client_header_buffers directive in the http block of your Nginx configuration file. For example:
http {
# Your http block directives go here
# Increase the size of the HTTP header buffers
large_client_header_buffers 8 16k;
}
In this example, the large_client_header_buffers directive is set to 8 16k, which means that Nginx will handle HTTP headers up to 16 kilobytes in size. You can adjust the value of this directive to match the size of the headers that you expect to receive from the upstream server.
Once you have modified the large_client_header_buffers directive, you will need to restart Nginx to apply the changes.
For more information on the large_client_header_buffers directive and other ways to optimize Nginx performance, you can refer to the Nginx documentation.