Kneepy

ошибка была связана с CORS и необходимо было указать некоторые параметры:

const io = require('socket.io')(server, {
   cors: {
      origin: "http://localhost:8080",
      methods: ["GET", "POST"]
   }
})

Кому интересна судьба этого запроса, то я пришёл к такому решению

SELECT
             p.title, p.supplement, p.link, p.file, uPost.avatar, p.pid, uPost.lastname,
             CONCAT(IFNULL(uPost.firstname, ''), ' ', IFNULL(uPost.surname, '')) AS name,
             uPost.uid, 
             GROUP_CONCAT(distinct JSON_ARRAY(
                 JSON_OBJECT(
                   'name', CONCAT(IFNULL(uComm.firstname, ''), " ", IFNULL(uComm.surname, '')),
                   'avatar', uComm.avatar,
                   'lastname', uComm.lastname,
                   'value', c.value,
                   'cid', c.cid, 
                   'isAnswer', IFNULL(c.answer, false),
                   'answers', (SELECT GROUP_CONCAT(JSON_ARRAY(JSON_OBJECT(
                   		'name', CONCAT(IFNULL(uComm.firstname, ''), " ", IFNULL(uComm.surname, '')),
                       	'avatar', uComm.avatar,
                       	'lastname', uComm.lastname,
                       	'value', commAnswer.value,
                       	'cid', commAnswer.cid
                   ))) AS answer FROM comments commAnswer WHERE commAnswer.answer = c.cid)
                 )
             )) AS comments
           FROM posts p 
             JOIN users uPost ON p.uid = uPost.uid AND p.pid = ?
             LEFT JOIN comments c ON c.pid = ?
             LEFT JOIN users uComm ON uComm.uid = c.uid