Within the Tools>Site Health section you will notice such an error caused by the Unyson plugin:
"A PHP session was created by a session_start() function call. This interferes with REST API and loopback requests. The session should be closed by session_write_close() before making any HTTP requests."
In order to fix it, you will need to add the following code within the functions.php file of your child theme:
if (!function_exists('_disable_fw_use_sessions')) { add_filter('fw_use_sessions','_disable_fw_use_sessions');
function _disable_fw_use_sessions(){
return false;
}
}