The firmware are region specific and I do remember downloading these from firmware.denon.eu or some such denon eu website.
If your AVR is not facing any problems, I strongly suggest that you let it run with the current version.
I do not know if these are the latest firmware, but I have two files in my archives. I am not sure if these are working versions as I am not using X1200w anymore. I also do not recollect if I ever used either of these files for an USB update.
USB_AVRX1200E2_CY920_000100820200-0009.zip
USB_AVRX1200E2_CY920_000100820200-0012.zip
Consider using these files if and only if you have no other options. I will share them once you confirm and you can provide me with a way to share these with you. Both files are around 27MB in size.