I installed again and, on completion, I noticed when I clicked on the back-end button there was a link and a message that my admin area had a new url.
The previous time I deleted the install directory too early and didn't see the message.
This appears to be solved. I assume you just use the new link form this point on and that it was generated for security. It would be good to make that more clear during installation to avoid confusion.
Just a suggestion. Aside from that, it looks like a great platform with many features.
jollyfrog's post in Admin area: Page not found was marked as the answer