Verify keys are not too similar

so this it the mitigation that @udidahan recommended. the problem is that not  being able to reproduce the issue (cant find too keys that cause the problem) i dont know if the "string being encrypted" is a variable that impacts the outcome.

Can anyone come up with anything better?

For reference here are the SO articles 

 * http://stackoverflow.com/questions/1556683/does-rijndael-always-throw-an-exception-if-the-password-isnt-correct
 * http://stackoverflow.com/questions/5812232/c-sharp-aes-rijndael-detecting-invalid-passwords
I think we need to better explain to the user why this exception is being thrown and how to fix it.
Something like, "The new encryption key is too similar to the existing Expired Keys, This can cause issues when decrypting data. To fix this issue please ensure the new encryption key is not too similar to the existing Expired Keys".