XF 2.1 FastCGI error

[Nuno]

Hi

From time to time I get this error in the logs, but I don't understand what is causing this issue:

Jul 20 11:19:04 server example.com 2021/07/20 11:19:04 [error] 2962014#2962014: *1138474 FastCGI sent in stderr: "PHP message: array (
Jul 20 11:19:04 server example.com   'version_id' => 22,
Jul 20 11:19:04 server example.com   'version_string' => '2.2',
Jul 20 11:19:04 server example.com )" while reading response header from upstream, client: << client ip >>, server: www.example.com, request: "POST /register/register HTTP/2.0", upstream: "fastcgi://127.0.0.1:9002", host: "www.example.com", referrer: "https://www.example.com/register/"

How can I debug this?

Thanks