Domain Mapping goes wrong

Hello hello

I need some assistance, lets say I have http://www.mysite.com/stuff

Where users can create blogs at the url http://www.mysite.com/stuff witch will be then user.mysite.com/stuff

Now when ever you want to map a domain to that blog it turns out to do this http://www.myothersite.com/stuff where it should only be http://www.myothersite.com

How could I fix this, help help I am drowning :wink: