Dear dnn-fellows

I want to create a child portal. dnn version 4.3.4. But I can't. Am I so stupid?
I've tried everything I think...

I login as host
I go to host
I go to portals
Add new portal

Then I've got the following error:
"The Child Portal Name You Specified Already Exists. Please Choose A Different Child Portal Name."
 
Then I found this thread http://forums.asp.net/1153897/ShowPost.aspx
Where they said that in my portal alias I must eliminate the last /:
Portal alias: www.mywebsite.net/MyChildPortalName.
I've tried this but then I've got the following error:
"Violation of UNIQUE KEY constraint 'IX_PortalAlias'. Cannot insert duplicate key in object 'dbo.PortalAlias'. The statement has been terminated. "

I've tried to customize the home directory
Home directory: MyChildPortalName

Then I've got the following error:
"The Child Portal Name You Specified Already Exists. Please Choose A Different Child Portal Name."

In the Professional DotNetNuke 4 book from Schaun Walker, Joe Brinkman, Bruce Hopkins, Scott McCulloch, Scott Willhite, Chris Paterra, Patrick Cantry, Dan Caron there you can find at page 86:
Notice the radio buttons for selecting a parent or child portal. Selecting the child portal requiers no further configuration outside of the application.

But it doesn't work... What have I to do so it could work.