Hello there @rmrodrigues and welcome to the community!
Would you be able to please send this over to appsupport@monday.com with an explanation on which webhook you are receiving (with which event) and the app ID, so we can take a closer look from there?
How are you generating the webhook? You should be generating it when you install/uninstall the app from the “install” section (on the left menu, one item below “webhooks”).
Only other “obvious” thing is confirm you’re using https not http for the URL. I don’t think that would cause this for the body, but its only thing I can think of that could cause it. Beyond that its either some sort of issue on the monday side, or something odd on your backend.
If you find an answer let us know, in case anyone else encounters it.