NGINX. Фиксим "connect() failed (10061: No connection could be made because the target machine actively refused it) while connecting to upstream"

    Несколько недель тому назад я заметил, что у одного из наших заказчиков NGINX спамит в error.log ошибки начинающиеся с "connect() failed (10061: No connection could be made because the target machine actively refused it) while connecting to upstream...":

2020/11/20 19:47:24 [error] 8560#14660: *1247 connect() failed (10061: No connection could be made because the target machine actively refused it) while connecting to upstream, client: 111.111.111.111, server: , request: "GET /favicon.ico HTTP/1.1", upstream: "https://XYZ.XYZ.XYZ.XYZ:3000/favicon.ico", host: "domain", referrer: "https://domain/"
2020/11/20 19:47:59 [error] 8560#14660: *1247 connect() failed (10061: No connection could be made because the target machine actively refused it) while connecting to upstream, client: 111.111.111.111, server: , request: "GET /ru/search?cs=N4I...

где XYZ.XYZ.XYZ.XYZ – это IP адрес их сайта, а domain – его доменное имя.