How to increase client request header buffer size due to 400 Bad Request errors

For Nginx, when a client sends a request line or request header, the directive client_header_buffer_size is used, which allows for header lines or requests to be a maximum of 1024 bytes, by default. This is sufficient for most requests. However, if a request includes long cookies, or comes from a WAP client, it may not fit into 1024 bytes. If a header line or request does not fit into this buffer then the directive large_client_header_buffers is used, which by default is 8190 bytes.

For Apache, by default, the directive LimitRequestFieldSize is used to determine the max size of a header lines or request which is 8190 bytes.

Note: When name-based virtual hosting is used, the value for this directive is taken from the default (first-listed) virtual host that best matches the current IP address and port combination. Plesk uses name-based virtual hosting only. Therefore, if your domain is using Apache, you must apply this globally as you cannot apply it to a single domain alone.The only way around this is to use Nginx-only hosting for the domain in question.

PLESK_WARN: Due to the above note, the preferred solution is the Nginx-Only Hosting option. As applying this globally increases the potential for the server to be affected by DDoS attempts, as it increases the allowed header request size for every domain on the server.
Even if Apache and Nginx are used together, because it's possible to bypass Nginx and reach Apache directly on port 7080 & 7081.