Hi
From what you've written I would assume that you are handling your Log Out event by check IsPostBack in the Page_Load method on your masterpage. This would not be the correct way to go about it, as it now hijacks all button clicks.
Instead rather wire up the button to a specific event handler in your master page code behind.
For example:
MasterPage:
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
MasterPage code behind:
protected void Button1_Click(object sender, EventArgs e)
{
//Run your log out code in here
}
This should prevent your master page for hijacking your other button click events. Hope this helps.