Any time it will be happen when working with file upload control and the best solution is to use update panel to reduce this kind of error. I hope its will be working properly...:-)
<table width="100%" align="center" cellpadding="0" cellspacing="2">
<tr>
<td class="style2">
Enter Access Path :
</td>
<td>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="cmd_getpath" runat="server" OnClick="cmd_getpath_Click" Text="Get Path" />
<asp:TextBox ID="txtPath" runat="server"></asp:TextBox>
</ContentTemplate>
<Triggers >
<asp:PostBackTrigger ControlID="cmd_getpath" />
</Triggers>
</asp:UpdatePanel>
</td>
</tr>
</table>