PayPal Sandbox testing of IPN Listener always returns INVALID

[pp2017acc]

I am working on a custom payment system for my blog (PHP), and now I am trying to implement the IPN system.

I have fully copied the PayPal php example, but no matter what I do, I always get an INVALID response when validating the IPN request.

I have read tons of other topics explaining the same problem and still can't find a solution.

- I am using the OFFICIAL code example

- I am using the sandbox IPN address

- I have tried with and without setting a value in 'payment_date' , because many others suggested that the problem is in the plus sign inside the date.

- Tried with and without https:// at my end

- I have tested the code on 2 different servers, from 2 different PayPal developer accounts.

And each and every time the response of the validation is INVALID.

I am almost ready to give up...

Thanks in advance!

[pp2017acc]

There are currently 4 topics with no reply in this section, all about this specific problem.

At least some of your tech team, can step up and confirm the problem?

[MLRTdirector]

I've even emailed myself the message sent by Paypal and my return message to verify myself that the message is the exact same. I get a green light from the IPN Simulator but the response is always INVALID.