Sorry to bump an old topic, but it has high ranking on Google and no resolution. I ran into this problem with my HG610 and successfully recovered from it, so I wanted to post how to fix it.
It seems it occurs when the NVRAM is corrupt. No amount of reflashing or factory resetting will help.
What you need to do is to open it up and connect to the onboard serial header (TX, RX and GND). You'll need a USB to UART adapter which can be had for quite cheap off eBay. Make sure it is 3.3V and not the old standard RS232 12V or you'll cook the device.
Once connected, use some sort of terminal software (I used RealTerm) and choose the USB device. Speed is 115200-8-N-1 and display is ANSI.
Power on the modem and you should see the boot loader. If you see readable text, make a copy of the details shown. If you see gibberish (or nothing) then you're probably not connected correctly or your speed is wrong.
You should get an option to interrupt the startup process by pressing a key, so press one. From here, one of the available options is to erase the NVRAM, so do that.
Next, it will restart and ask for those details you made a copy of earlier. Type them in carefully matching the original values.
Now we boot the modem like normal and it begins to start up, although if we pay attention to the Linux boot messages, we see a few errors. That's because we still need to give the OS a few details which can be done by logging in, going to the shell (sh) and then following the directions for setting serial number, board number, and MAC address here:
https://huaweihg612hacking.wordpress.com/2011/07/17/the-nvram-region-of-the-hg612-flash-memory/We seem almost done, but a final factory reset (hold the button on the rear for a few seconds) will finish up the remaining missing pieces.
Finally, the device reboots and it comes up just like new. And BTW, if you can't login to the web interface as admin, try using Internet Explorer (yuck) as it seems to work best.