PayPal - PHP API, custom webform, unsuccessful payment without any errors - php

Question

I've got a webform, sending data (and the customer) to PayPal.
My JSON request:
{
"intent":"sale",
"payer":{
"payment_method":"paypal"
},
"redirect_urls":{
"return_url":"http://dev1.url.de/payment/execute?method=paypal&price=1&order_number=123465&success=true",
"cancel_url":"http://dev1.url.de/payment/execute?method=paypal&price=1&order_number=123465&success=false"
},
"transactions":[
{
"amount":{
"currency":"EUR",
"total":"1"
},
"item_list":{
"items":[
{
"name":"123465",
"currency":"EUR",
"quantity":1,
"price":"1"
}
]
},
"invoice_number":"123465"
}
]
}
and a PayPal response:
{
"id":"PAY-0YV50781X7702245FKVDWC5A",
"create_time":"2015-05-04T12:09:24Z",
"update_time":"2015-05-04T12:09:24Z",
"state":"created",
"intent":"sale",
"payer":{
"payment_method":"paypal",
"payer_info":{
"shipping_address":{
}
}
},
"transactions":[
{
"amount":{
"total":"1.00",
"currency":"EUR",
"details":{
"subtotal":"1.00"
}
},
"invoice_number":"123465",
"item_list":{
"items":[
{
"name":"123465",
"price":"1.00",
"currency":"EUR",
"quantity":"1"
}
]
},
"related_resources":[
]
}
],
"links":[
{
"href":"https://api.paypal.com/v1/payments/payment/PAY-0YV70781X7192245FKVDWC5A",
"rel":"self",
"method":"GET"
},
{
"href":"https://www.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-38Y65870AE087743W",
"rel":"approval_url",
"method":"REDIRECT"
},
{
"href":"https://api.paypal.com/v1/payments/payment/PAY-0YV50781X7170245FKVDWC5A/execute",
"rel":"execute",
"method":"POST"
}
]
}
And going through the payment process on the PayPal website seems all to be fine.
I can login, I can see all correct details and after I click on send payment, it redirects me to my form (return_url) and it says I paid.
The thing is .. it's not paid.
I can't see the money sent and it never arrives - not transaction is triggered. How can I figure out the problem? Or does anybody have a clue?