I went with the suggestion from another thread, which was to amend our code to loop up to three retries from our server to the IPN URL if the response from IPN is not "VERIFIED". We shouldn't have to, but it has been working so far. On the other hand, my endpoint monitor which hits IPN with a blank GET request every 61 minutes has still received the 400 Bad Request response around once per day on average, so I'd hate to think how often people with a busy integration are seeing this error if I'm still seeing it daily when hitting it that seldomly.
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We're seeing this issue too - for an online retailer that takes approx 1000 orders per month outside sale seasons. We've seen this error 4 times starting on 2nd July 2019. Zero occurrences before July and this integration has been live since 2015 and only previously interrupted by the occasional SSL change. 400 Bad Request Your browser sent a request that this server could not understand It does not appear to be specific to the content in our request to the IPN URL either - I am confident of this because I have an automated test configured that hits the IPN endpoint once per hour with an empty request and alerts me if we get back anything other than the expected "INVALID" response. This is intended to warn us to investigate SSL and network conditions on our side and to go check Paypal connectivity from another box, however it has also managed to report the 400 Bad Request error a few times this month, always resolving itself on the next test attempt, despite no changes on our side.
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