As best I can tell from the error payload in my developer console, Dropbox is actively disabling my webhook endpoint for returning a 202 Accepted in response to the incoming "notification request".
It looks like I misinterpreted the documentation that states any "non-200" code will be treated as an error to mean any "non-2xx" code will be treated as an error. I will stand by the fact that it seems odd that a 202 is handled as an error here as it feels like the right response, but I can update our code to return a 200 instead.
Thanks for following up. Yes, the Dropbox webhooks system expects a 200 specifically, but I'll pass this along as a feature request to allow 2xx. I can't promise if or when that might be implemented though.