In their brand new A9LH guide they say this :
"Problem:3DS won’t turn on after using Gateway Time-Machine to downgrade to 2.x
Solution:Remove the internal SD Card and power on your system, then re-insert it after it boots and continue the guide"
But when I go to the go-gateway site my 3DS crash...
I found the solution, well, at least for my case. It was almost the same as your case.
"My english isn't the best, but I will try to explain it.
In the middle of a downgrade, (from 4.5 to 2.1) something failed, and the message "an error has ocurred" appeared.
I was in despair, I tried everything, I searched everywhere on the NET. I tried the page go.gateway-3ds, and a thousand of pages like that to run the exploit with the Gateway's launcher.dat. Then I tried to install rxTools 3.0, but It didn't worked.
So, I was thinking that, if the pages run the payload automatically, they may be running the wrong payload. Because I had a combination of 4.5 and 2.1.
I installed an APP from Playstore, (thinking that the app would let me choose the payload, but it didnt), and hosted the launcher.dat of gateway. I inserted the IP that the app gave me on my 3ds, and it worked. I was on the Gateway menu. The page go.gateway never worked, but the app worked. Strange, huh?
The same app, had an option to run rxTool.dat as well, and it worked. (The official pages to install rxTool didn't worked). And that's pretty much how I did it. Then I just installed FBI.cia and sysupdater.cia, I downloaded the 9.2 files of USA and updated. Now everything is working smoothly.
Sorry for the broken english.!
The APP is 3ds x loader