I just tested something.
Clicked on "Sign up now" while I was logged in here (aka avast account)
It took me to
https://business.avast.com/ where I (ofcourse) automatically logged in.
I too got the "Are you sure that you have the right address? Sorry, we can’t find the page you referenced." message.
But I was able to continue without a problem and after clicking "Add device", I was able to download the client installer.
Both the online as well as the offline version.
It seems that if you signed up before avast withdraw the free version, you still can use it.
If I am not logged in and click on the sign-up link I am taken to a form where I can create a account. (see attachment)
I haven't created a fake account to see what happens, but it should give you the option to use AFB for free as you get there through the page where it clearly says
Avast for Business is free to use for as long as you want and for an unlimited number of admins and devices
If not, avast is for sure violating the Dutch laws (and likely laws in other countries as well).