The default Plesk configuration runs a mix of both Nginx and Apache to serve files. For this configuration Nginx acts as head server, where it's able to serve static media directly and then proxy all PHP (and similar) calls through to Apache. This allows the systems to take advantage of the speed and features of Nginx (such as HTTP/2) but retain compatibility with applications which have a .htaccess file or other Apache specific configurations.

For certain configurations where there are large cookies or other server headers set, this can cause the default proxy buffer to fill and generate an error. This error will look something like:

upstream sent too big header while reading response header from upstream

To fix this, you can simply increase the proxy buffer sizes for Nginx, using the instructions below.


  1. Login to Plesk and select your domain. 
  2. Under Apache and nginx settings, add the following to the Additional nginx directives
    proxy_buffers 8 16k;
    proxy_buffer_size 32k;

    Your configuration should look like this:
    plesk - additional nginx directives - proxy buffer increase

  3. Click OK to apply.
  4. Monitor the error logs to ensure this has corrected the issue.
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