@olivierlambert I know this topic is quite old, but I'm having this problem as well. My XCP-ng version is 8.2.0 and one additional detail is that I already have an XO from sources instance running on this pool. I just wanted to install the official free XOA to check out what it offers and what it doesn't.
Both the Quick Deploy button on the webpage of the host, and the Deploy form on the XO website don't work.