System.Security.Cryptography.CryptographicException: Store Root doesn't exists.

I've seen this error only once or twice in lots of testing.

The only https url I'm using is a https://maps.googleapis.com/ one and it works most of the time (99%).

But just a few debug runs have failed with the exception sequence:

System.Security.Cryptography.CryptographicException: Store Root doesn't exists.
System.Security.Cryptography.CryptographicException: Store CA doesn't exists.
Mono.Security.Protocol.Tls.TlsException: The server stopped the handshake.
System.IO.IOException: The authentication or decryption has failed.
System.Net.WebException: Error writing request: The authentication or decryption has failed.
System.Net.WebException: Error getting response stream (Write: The authentication or decryption has failed.): SendFailure

Currently seeing this very rarely on a Nexus running JellyBean

I've not got this easily or predictably reproducible - sorry. Re-running the same app (the same install - no system change made) just seems to work :/

Any ideas?

Stuart