About PostBackUrl of BUTTON

Dear all,
I am using C# to develop website.
In the BUTTON's  [PostBackUrl], I tried:
"~/FG/Default.aspx"   and  "/FG/Default.aspx"
The result is: both are work in my LOCAL HOST ( after click the button, the webpage can jump to  FG/Default.aspx )
However, after I upload them to my website, both of the above solution cannot work ( after click the button, the webpage is refreshed but do not jump to  FG/Default.aspx )
So, I am suffering to write programming to every BUTTONs which are needed to jump to other pages, the progress is like:
                         protected void Button2_Click(object sender, EventArgs e)
                     & ...