You can also view all events from the Beebox user interface. Whenever a callback URL is configured these pages show the exact url being called as well as a status. If the URL did not respond then you will see an error information.
Connect to the Beebox user interface, select a project and click the small "Recent events" link below the left navigation:
You can also view events for all projects combined. Click the "Account" link in the top navigation menu, then "Administration" and "Recent events":
If the callback URL is invalid or does not respond, an error message is shown: