Why does my Facebook Webhook Subscription Verification timeout on my ngrok URL?

_x000D_ _x000D_ I've been using ngrok for several weeks while building the first version of my Web Server so that I can send it test Webhooks from a Facebook App. Until today, I could always just update my Webhooks subscriptions with the new auto-generated ngrok URL just fine. But today, when I try to verify my ngrok URL with my Facebook App's Webhooks, I do NOT get an indication from my server that it ever received a GET request, I keep getting the timeout message: The URL couldn't be validated. Callback verification failed with the following errors: curl_errno = 28; curl_error = Operation timed out after 6001 milliseconds with 0 out of 0 bytes received; HTTP Message = Unknown Response Code However, when I use Postman to hit the same ngrok URL, everything works fine - I can see that I'm getting back the challenge string and my server indicates that it received the GET request and responded with a 200 code. I haven't changed the logic of the verification endpoint in my server since I last used the Facebook Webhooks (yesterday). Graph API version is 2.9. I made sure I was using the HTTPS URL ngrok provides, as I have been doing.