The only available L3+ firmware on Bitmain's site is: Antminer-L3+-201811131346-384M.tar.gz
If you update to it it adds the "Cannot find Signature" so no going back or going to Bliss.

I suspect that any Bitmain firmware for any miner from Nov/Dec 18 that says "Security Firmware, Fix some security vulnerabilities"
blocks unauthorized firmware, and rolling back to previous firmware.
Even resetting does not get around it.
 
Thanks for this work around.   

-M