The backend service is running at the specified URL, and token validation is in place. Previously, I was able to verify the webhook successfully, but now I am encountering an error.
Try a forum search first please. Some ngrok domains seem to be blocked by FB.
I have that problem too.
In my case I have a webhook configured with ngrok for WhatsApp but I couldn't verify the callback URL anymore.
I was using the free version of ngrok but I tried the paid version to get the domain and it solved the problem. I hope it helps you!