Getting a 500 error when trying to initialize SDK

Getting a 500 error when trying to initialize SDK

I've managed to successfully call API from Postman and get the right results, so I think I've set up all the tokens appropriately. But I am getting an error when trying to use SDK.
  1. 500 | Internal Server Error | com\zoho\crm\api\exception\SDKException
    stack trace at ()
    in SF_ROOT_DIR/vendor/zohocrm/php-sdk-2.1/src/com/zoho/crm/api/Initializer.php line 79 ...
                }
                catch (\Exception $ex)
                {
                    throw new SDKException(Constants::JSON_DETAILS_ERROR, null, null, $ex);
                }
                self::$initializer = new Initializer();
I suspect I didn't initialize properly, can you please help me out? I attached my initialization class. And I'm pretty sure it's the initialization (maybe proxy, do I need that?), because the code execution stopped in the initialization, before I got a chance to get an instance of RecordOperations Class in frontend.

Additional details:
* I used sdk v5.0.0 as the initialization is made simpler by reducing the required entities.
* Registered a Self client. Worked nicely from Postman.