2020/06/26 19:58:46 [error] 7957#7957: *5277925 FastCGI sent in stderr: "PHP message: PHP Warning: stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:
error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed in /app/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php on line 94" while reading response header from upstream, client: z.z.z.z, server: ~^(?<flarum_tag>.+?)\.flarum.cloud$, request: "POST /register HTTP/2.0", upstream: "fastcgi://unix:/run/php/php7.2-fpm.skilleddevs.sock:", host: "skilleddevs.flarum.cloud", referrer: "https://skilleddevs.flarum.cloud/"
I don't know why, but you have set mail encryption to TLS. Don't do that.