The full and permanent link (until the next firmware release) is on the main Denon DJ website.
The temporary link given in the forum acouple of weeks ago (now removed) was put here to give forum users a benefit of accessing the new firmware a day or so before the mainstream public link and downloads started.
I’m having this problem, too, but the above fix doesn’t work. Run into a dead end when loading the X1800 Prime Firmware Updater software. Only option is “Exit” button. I click exit, nothing happens. Got a call into support, but they are backlogged today and someone had to take my message.