I don’t think php-fpm is the problem.
Remove the {"request_data": [ at the beginning and the trailing "]}
You can use the inside syslog-function in api_users.class.php in the function post(…) ala:
dol_syslog(get_class($this)."::post ".$this->useraccount->$field." value: ".$value, LOG_ERR);
insert before $this->useraccount->$field = $value;
and look at the results in dolibarr.log if you have the debug-module activated.
Here a screenshot from the Swagger UI
→ user “XYYY” is successfully created
