Hello, I was having the same problem and fixed it. In this case it was because a forum user (admin) was registered with the same email than a joomla user (juanpablo -superadmin-), so when the script tried to create the joomla user admin with the email juanpablo@domain.com it gave an error, because my joomla  superadministrator user (juanpablo) is already registered with that email. So I changed the email for user juanpablo (under "user management") and it worked

Have a nice day

Juan Pablo Olivera