400 Bad Request

Hey! I've used this codebase to put up a quick testing app: https://www.esignlive.com/blog/esignlive-how-to-creating-a-simple-php-web-application/ Unfortunately I am stuck for a day or so with a 400 error at this point:

POST /api/packages/A8e11I57ytWC1GHJzzphfLISw04=/documents/signed_documents HTTP/1.1
Host: sandbox.esignlive.com
Accept: */*
Authorization: Basic api_key
Content-Type: application/json
Content-Length: 138

The package is created successfully in the sandbox account, the signer is added as well but it cannot be set to SENT, as it yields a Bad Request error. I've tried changing the document and the API endpoint, but no luck... One thing that I noticed is that even in the admin panel it cannot be sent without adding the dynamic signature field in the document, am I missing something?