I find it odd that you state that the user name was already used and you say it is not.
There are two names that need to be unique.
User name and display name.
The new registrant may not notice the difference on the error message.
But there are separate messages for each.
But if you can get that info you can search for both in member management.
It could be the display name was taken and not user name.
For this part I can't see that as a browser/firewall/virus protection issue.
Also it has been suggested to turn off the referral check for several years.
With today's standards it causes more harm than good.
