Core issue
This error occurs due to old RSA keys that are present in the PIX Firewall configuration.
Resolution
In order to fix this issue, regenerate the RSA keys in the PIX Firewall with these commands:
Pix(config)#ca zeroize rsa
Pix(config)#ca generate rsa key 512
Pix(config)#ca save all
Pix#write memory
Pix#reload