Permission failure on button click

[Guest]

We have an aspx and aspx.cs.

The aspx has
<asp:button id="btnCancel" onclick="btnCancelEvent" Runat="server" Text="Cancel" Font-Bold="True" Font-Size="8" Font-Name="verdana"></asp:button></TD>

and the aspx.cs
public void btnCancelEvent(Object source, System.EventArgs e)
{
//Handle the cancel button click event
ViewControls(false);
lblMsg.Text = "";

}

and

private void Page_Load(object sender, System.EventArgs e)
{
// Put user code to initialize the page here
if (!IsPostBack)
{
GetData();
ViewControls(false);
specialSecHeader.TitleText = "page title";

}
userName = User.Identity.Name;
if (userName.IndexOf("mydomain") > -1)
{
string myString = userName.Substring(10);
userName = myString;
}
}

When the cancel is pressed it comes up with a connect window asking for user name and password to connect to websvr.xxxx.com).
The same thing happens with another button handled in the same way (save).

To display this page it has had to execute an addnew which looks like very similar functionality.

I am told this was all working on Friday but not now. No one owns up to changing anything but...
The code in question hasn't changed but anything else could have.

Anyone any idea about what to look at?