"an unknown error occurred while validating the service account"
and the installation would not complete, and I also got a configuration error with something like "unable to configure service".
After googling a bit and playing around with the settings, the answer was pretty simple. The default administrator account needs to be enabled. Although this was a pretty easy answer, I couldn't find it anywhere!
|Result after enabling the default administrator account|
Sorry I don't have more screen-shots, but I think it's pretty self explanatory. I may or may not add more later.