Private Key Warning Dialog

If your application is using a pre-installed certificate for creating a digital signature, or for decrypting, then it needs access to the private key. (By pre-installed, we mean a certificate that has been imported from a PFX (or via the browser) into a Windows registry-based certificate store.)

If you chose to enable strong private key protection, then the Windows operating system will display a warning dialog when any application attempts to access the private key. To disable this warning, you’ll need to re-install the certificate without strong private key protection.

This is what the warning dialog looks like:
Private Key Warning Dialog

When importing the PFX via the Certificate Management Console (cert.msc), do not check the checkbox to enable strong private key protection:
PFX Import