I flashed my WRT54G (v3) from a custom firmware to the lastest Linksys one (v4.21.1)

Now the internet/LAN works fine, but the web interface (http://192.168.1.1/) shows broken pages.

I tried to manually navigating to the firmware upgrade page (by entering its URL), but it is broken too and not working.

I tried a 30/30/30 reset twice, but the broken web interface still shows up.

I tried to flash using the tftp2.exe application, which told me that the router didn't respond to the request.

I tried to flash using the Windows commandline TFTP protocol, but I don't know how to supply a password on this prococol. (there is a factory default password ("admin") which I can't remove since the web interface is broken)