Not sure if I solved this or it's just coincidence, but mine is now working again, still on the same Roku power adaptor, no error messages. I had to reset it using the recessed button on the back something like a dozen times though, the last 3 or 4 I tried to input the network ID and password manually before the low power message came back, on the 4th time I beat the message and the Roku came back to life, that was 4 days ago and it's still working.
Not sure if it fixed itself or what I did fixed it, but worth a go, you do need patience though!
Until I managed to get the network details in manually the MAC address was a bunch of zeroes (which won't work with any network) and the automatic scan for networks was dead. Once it started working all went back to normal, proper MAC address etc (must be a virtual one in software).
Not sure if firmware update to blame as last software update still shows as 18 December and it was about 3rd Jan when it stopped working, unless it's somehow reveted to a previous one.
Obviously this hasn't happened to you. When the low power error message comes up you can't get past it. It just locks up on the screen. There is no way to get to the settings menu because the unit will not power up to full and just sits there with the error message on the screen.
It took me several resets (via the recessed button on the rear of the Roku) plus leaving it unplugged overnight a couple of times, about 3-4 days total. Then 4-5 unsuccessful attempts to get to the info before the low power message appeared. (you have to be fast on the on screen keyboard).
I am having the same issue. I am using the provided cord and power adapter and I keep getting the low-power message. There are times when it will not even start up to get the message; I have to unplug it completely for 20 seconds and restart it. I did try a different adapter, but receive the same message. I will try and different power cord and see what happens.