Old question, but I ran into it too. In my case, it was because the private key file was readable/writable by all users on my local (Windows) machine. To fix this

1. Right-click on the file and choose properties

2. Click on the security tab

3. Click the "Advanced" button on the lower right

4. Remove any users or groups other than yourself and the System Administrators

5. Add yourself back into the list and make sure you have at least read privileges