Response.Redirect behaves different from within UpdatePanel

Everywhere in my web application i use redirects like these:

Response.Redirect("~/SearchResults.aspx", true); 
 

And this always takes me to the right page. http://localhost/myapp/SearchResults.aspx

But now i'm doing this in the onclick event of a button that sits in an ASP.NET UpdatePanel and it tries to bring me to the following address:
http://localhost/myapp/%2fmyapp%2fSearchResults.aspx

Does anyone have an idea how to fix this?

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