Fiddler has been my go-to web application debugging tool for as long as I've needed one. Yesterday - it simply decided it didn't want to decrypt any more SSL traffic for me. No settings changed (no, really) or anything and I started seeing this error for every site I tried connecting to:
fiddler.network.https> HTTPS handshake to www.google.com failed. System.Security.Cryptography.CrytpographicException Cannot find the requested object.
Not a particularly helpful error message (at least not to someone who isn't the developer) so off to Google I went. A few hours of searching through various Google Group discussions mentioned some recent changes to the .NET framework which has changed certain cryptographic behaviours and broken things. The solution is to go to C:\Users\[your username]\Documents\fiddler2 and rename the file ClientCertificate.cer to something else. I'm not 100% certain on why this certificate would cause the problem in the first place but it fixed my SSL interception so I'm happy again. Full discussion from here.