Firstly, although the latest US firmware was working just fine for me, I take no responsibility if something wrong happens to your router. I don't work for Buffalo.
I'm just like you, I came here for help but, unlike you, no one helped me :) .
Assuming you are running Windows 7:
1- You have to be wired, and plugged to one of the four Ethernet ports (black). WAN port (blue) & Wireless won't work here.
2- Fully disable Windows firewall, or whatever firewall you are using.
3- Give your Ethernet network adapter an IPv4 static address by going to the adapter properties:
(IP: 192.168.11.2) & (Mask: 255.255.255.0) & (Gateway: 192.168.11.1)
4- Get the index number of your Ethernet network adapter by running:
route print
Then, in "Interface List", in the far left you will find the INDEX number and In the far right the adapter name.
Get the INDEX number corresponding to your Ethernet adapter. (Mine is 11 for example, yours will be different)
5-Make the router MAC address static and map it to router IP address.
Replace __INDEX__ in the following command with the one you found from the previous command.
(Please Run the command line as an admin: Click Start, write "cmd" in the search field, Right-Click on "cmd.exe", then choose "Run as administrator")
netsh interface ipv4 add neighbors __INDEX__ 192.168.11.1 02-AA-BB-CC-DD-1C
6- Disable "Media Sensing" feature:
http://support.microsoft.com/kb/239924 (Info and howtos about this feature)
http://go.microsoft.com/?linkid=9742317 (Direct installer link to do it for you)
7- Enable TFTP client in Windows by clicking Start, and typing "OptionalFeature", without quote, in the search field.
8- Reset router settings.
9- Now make the following command ready, DON'T press enter now.... WAIT:
TFTP -i 192.168.11.1 Put "The-full-path-of-your-downloaded-firmware-with-quote"
10- Unplug the Power....
11- Plug in the power and start counting to 8 seconds then immediately press Enter in that previous TFTP command.
12- Diag will be flashing for a few seconds, the router will reboot.