Unable to post to a test connector's webhook url

After setting up a connector pretty much as described in the connector tutorial, we're not able to send test requests to the webhook url we received from the connector configuration page.

1. The test connector is set up in the Connectors Developer Dashboard. Not published to the app store, as still testing. The message next to the connector name says "Registered. Pending Approval"
2. It's sideloaded to Outlook / Microsoft Teams
3. It's also uploaded separately as a package into Teams
4. It shows up correctly in the channel configuration
5. Connector Settings page loads correctly, and settings can be saved
6. The getSettings call returns a webhook url correctly
7. But, submitting a POST request to the webhook url errors out:

curl -d "test=true" -X POST 'https://outlook.office.com/webhook/09f2b8e5-e684-4e87-8838-75e3a2f9dcbc@00d35244-2e42-4af7-a1ad-d6b557249c2b/58f4ab6b-234c-43ad-82f0-4641e38699e0/c516d999fec5498e9cac06f8d0478f22/79e349e1-47da-487a-90ee-040eccbbfa8b' Unknown Connector provider 58f4ab6b-234c-43ad-82f0-4641e38699e0

This seems like an issue with testing webhook urls not being usable?

Related thread as another team is having the same issue as well, and github issue with more details.

As a side-note, when adding the connector to the team page, there was a chat notification that the user added the connector to this team. In this message the connector image did not load properly. I can see in the url that it also has the connector id in it.

This seems to have been an issue within MS Teams, and it looks like it has been fixed by now. I'm able to post messages to the generated webhook without receiving the error message.