add ReturnUrl but don't get redirected

Hi 
For a testproject I use two Login pages. I order to do that I use a dispatcher page:
Dispatcher:

/Login.aspx
Login-Pages:

/Admin/Login.aspx /User/Login.aspx
I then just redirect in the dispatcher to the real login-page according to the ReturnUrl. So far so good. The only problem I have is that I don't get redirected to the ReturnUrl once I'm logged in... Of course I add the ReturnUrl to the forwarded Login-Page (/Admin/Login.aspx?ReturnUrl=...).
What's the problem?
Any help is appreciated.
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