Hello! Sometimes we are getting 400 error from paypal when validating notification. But if i make the same request after some time it works correctly. Please check the response from paypal:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<title>400 Bad Request</title>
<p>Your browser sent a request that this server could not understand.<br />
I can not provide request, because it contains private information.
It happens only in 1-3% of requests.