@olivierlambert The issue was very much on my side (the network configuration was DHCP, so I blame my router). It was definitely not caused by XCP-NG itself.
I suppose the only thing that could potentially be refined on your side is the signposting, as the error message is a little cryptic. Having said that, the error says "source not found", which is absolutely accurate, and I guess there's no way for the installer to know why it can't find the source repository!
The main reason I posted was so the information would be available in this thread whenever the next person searches the same error, but thank you for following it up anyway. This forum is a great resource, and I'm sure I'm not the only one who really appreciates you taking the time to engage with the community and look for opportunities for improvement.