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@moshe007 in my case, I used PHP and my verify IPN method is based on https://developer.paypal.com/docs/api-basics/notifications/ipn/ht-ipn/ and using Sandbox, sometimes got a 200 response and sometimes 400 (like you).
The god news is that when I put my script in production, everything WORKS FINE!!!
Then it's obvious, Sandbox enviroment have problems with IPN.
I'm getting the same error. I'm using firebase functions to listen for IPN notifications, but I get "Bad Request Your browser sent a request that this server could not understand." as when verifying messages with paypal.