Dear Florian
We suspect that you are giving the wrong ID at:
$client->getTransactionService()->read($spaceId, $id);
You will have to pass on the ID which you have received in the Webhook.
If an empty result is still returned there, it is because the authentication is not working correctly.
Could you please review this? If this does not help, I recommend sending your inquiry to info@wallee.com in order for us to review this in more detail.
Kind regards
Patricius