Just for info.

When Updating a cart with more than six products, I was getting a 500 server error.

Server log error:
2024/03/11 11:25:49 [error] 15478#15478: *33608 upstream sent too big header while reading response header from upstream, client: 123.123.13.123, server: myserver, request: "POST /carro?action=update_product HTTP/2.0", upstream: "https://123.123.123.123:8443/carro?action=update_product", host: "myweb", referrer: "https://myweb/carro"
nginx is in use as a reverse proxy.

changing this value from 8k to 128k seems to have cured the error.

Code:
proxy_buffer_size   128k;
The only data being passed is the form data:
securityToken: d50ac3ffedac8cba699012ac9d9083a5
cart_quantity[]: 3
products_id[]: 4709
cart_quantity[]: 6
products_id[]: 6254
cart_quantity[]: 2
products_id[]: 4995
cart_quantity[]: 3
products_id[]: 4594
cart_quantity[]: 3
products_id[]: 4704
cart_quantity[]: 10
products_id[]: 4742
cart_quantity[]: 1
products_id[]: 4706
Seems strange to get such an error with so few products....