Hi Eric,
Not sure this is the right medium for a bug report, but I could not find a bug tracker for your Fiddler project. After update 2.6.1.5 I got certificate errors in browsers that said the certificate had an invalid digital key. After fiddling around I found that your HTTPS options tab changed. After switching to MakeCert things worked again. The issue can be reproduced by clearing the certificates from the stores and switching back to CertEnroll. Let me know if you need additional information or are unable to reproduce the issue. I am currently working on windows 8.1 and .NET framework 4.5.51650.
Cheers,
Jasper