It seems the certificate was not trusted for some reason. Would you please try the following workflow:
1. In Fiddler, navigate to Actions -> Reset All Certificates
2. Click Yes to delete the current certificates (if such) as prompted
3. A new informational window will pop-up. Press "OK"
4. Select Yes to Trust the Fiddler Root Certificate (as illustrated on the attached image).
5. Click Yes to install the certificate.
Please note you may need to restart the browser.
In addition, would you please clarify what is your operational system? Do you eventually test on a virtual machine?
What result do you get if you try accessing the following URL: https://platform.telerik.com/appbuilder/webclient/ping.json?