Recover from failed update

Hi,

I have an issue with an HTC nexus 9 tablet.
Here’s what happened. A while ago, I got a notification on the tablet, saying the Android 6.01 version was available for download and installation, so I went ahead and installed it.
It started downloading and installing.
It took a while it rebooted and suddenly the dead android appeared. I couldn’t do anything with it, except trying to reboot but when the device is rebooted the dead android is displayed again. So itś safe to say the device has failed to update properly.

I went ahead and jump onto google and see if I could find a howto on how to revive this tablet. Some suggested running a factory reset, so I did. No luck, still a dead android.
Others suggested wiping the data and cache partition and sideload a stock image by entering the hboot with the volume down + power switch and hooking the tablet up to a computer. I did that. I downloaded the appropriate stock image (volantis) and went ahead with the wiping of the data.
Then I was ready to load the stock image back onto the tablet. With the abd and fastboot tools.
I tried to do this, but it keeps telling me the boot partition is locked.

The device is seen by the fastboot tool, but not by the abd tool.

I cannot figure out how to unlock the boot partition now that I have wiped the device. I think it would have saved me al this trouble when I had unlocked the bootloader BEFORE I had started the OTA update, but I cannot change that anymore.

Can anybody help me out and get this tablet back to work?

Thank you in advance.