That error usually means there is a problem with the boot image. Were you running an insecure boot image when doing it or do you have a custom rom with an insecure boot image? Can you try rebooting or flashing a stock boot image (if you have a custom one) so that your system has the stock image and trying it again.
Pardon me but what is a boot image and what do you mean insecure? As far as I know, I haven't modified any boot image. I only flashed stock ROM, do a '1 click for all' using the toolkit, and flash a harsh kernel.
On fastboot, I have "Secure boot - enabled" and "Lock state - unlocked". Hope the information helps.
Edit: The toolkit says my boot image is secure.