Problem with postbackURL

hi,

I have one question regarding postbackurl.
I have one linkbutton on a firstpage (firstpage.aspx):
 <asp:LinkButton id="Linktest" onClick="Linktest1_Click" runat="server" PostBackUrl="~/testfiles/test.aspx"></asp:LinkButton>
 
And then I get these error:
 Unable to cast object of type 'ASP.nacrtovanepo_preglednovizunanji_aspx' to type 'ASP.nacrtovanepo_napovednpo_aspx'.
body {font-family:"Verdana";font-weight:normal;font-size: .7em;color:black;}
p {font-family:"Verdana";font-weight:normal;color:black;margin-top: -5px}
b {font-family:"Verdana";font-weight:bold;color:black;margin-top: -5px}
H1 { font-family:"Verdana";font-weight:normal;font-size:18pt;color:red }
H2 { font-family:"Verdana";font-weight:normal;font-size:14pt;color:maroon }
pre {font ...