I was developing Instant triggers for my Integromat app. I finished 2 of them, and when I started the third one, all webhooks stopped working. I wrote a mail to the support of about the situation and received the answer:
We can see in logs that webhooks fired from our end but your system returned us error 400 - it means that webhook works on our side but was not properly received on your servers side.
So why Integromat can answer 400 code? Is it a bug or my mistake? From that moment, I have tried with the new account, but the issue is still there. Any suggestions?
Judging by the response you received from the 3rd-party tech support, there's an error somewhere in your webhook code.
The first thing I'd check is of course the code directly inside your webhook's Communication tab, but also, if this code is calling any IML functions you wrote, you'll need to double-check that the IML code is not failing as well.
You could create an IML test using the Integromat Apps SDK VS Code plugin to simulate the function call with incoming JSON data (according to the documentation of the service you are implementing) and see if this call results in some kind of an unexpected error.