Symptoms

Error running K2 Designer throws the following error

"An error occurred trying to authenticate the user.

System.Security.Cryptography.CryptographicException: Keyset does not exist at System.Security.Cryptography.Utils.CreateProvHandle(CspParameters parameters, Boolean randomKeyContainer) at System.Security.Cryptography.Utils.GetKeyPairHelper(CspAlgorithmType keyType, CspParameters parameters, Boolean randomKeyContainer, Int32 dwKeySize, SafeProvHandleand safeProvHandle, SafeKeyHandleand safeKeyHandle) at System.Security.Cryptography.RSACryptoServiceProvider.GetKeyPair() at System.Security.Cryptography.RSACryptoServiceProvider..ctor(Int32 dwKeySize, CspParameters parameters, Boolean useDefaultKeySize) at System.Security.Cryptography.X509Certificates.X509Certificate2.get_PrivateKey() at System.IdentityModel.Tokens.X509AsymmetricSecurityKey.get_PrivateKey() at System.IdentityModel.Tokens.X509AsymmetricSecurityKey.GetSignatureFormatter(String algorithm) at System.IdentityModel.SignedXml.ComputeSignature(SecurityKey signingKey) at System.IdentityModel.Envelope"
 

Diagnoses

The error is generated because the SSL Certificate has been imported incorrectly into the Computers Certificate Store
 

Resolution

How to Import an SSL Certificate correctly:

In the MMC, add the Certificate Snap-in and add an instance of the Certificate snap-in for the Local Machine store.

Expand the Local Machine store and right-click on Personal, then pick the option to import a certificate, browse to the cert file (a .pfx in my case) and click next.

Enter password for private key (if exists)

Allow the cert to be placed automatically (this time it defaults to Local Machine - Personal)

Then you grant permissions using the "Manage Private Keys" option