I do have had this problem and believe it is related to the .sock file missing for the user.
2023/04/11 06:59:29 [crit] 5018#5018: *113 connect() to unix:/opt/alt/php-fpm80/usr/var/sockets/whocalau.sock failed (13: Permission denied) while connecting to upstream, client: 103.154.154.179, server: whocalled.au, request: "GET /wp-admin/plugin-install.php?s=w3%2520total&tab=search&type=term HTTP/1.1", upstream: "fastcgi://unix:/opt/alt/php-fpm80/usr/var/sockets/whocalau.sock:", host: "whocalled.au", referrer: "https://whocalled.au/wp-admin/plugins.php"
I rectified it temporarily by modifying my website .conf file to change the socket file to nobody.sock but it just means I lose some functionality in the backend of the Wordpress site, like not being able to install plugins directly and getting some permissions errors.
This is NOT a permanent fix. I would love to have a better solution.