Iframe not loaded with other browser than IE

Greetings!
I have an iframe in a web site that is not loaded when I try to view it in other browser than IE (IE works correctly).
The src attribute is set dinamically in the code behind.
The iframe is contained in an ascx page.

The web site is developped in ASP.NET/C# 1.1

Here is the HTML code:

<table id="Table1" cellSpacing="0" cellPadding="0" width="100%" align="center" border="0">
   <TR>
      <TD vAlign="top" align="right" width="15%"></TD>
      <TD style="WIDTH: 880px" vAlign="top" align="left">
         <asp:label id="SketchNoDelete" runat="server" Visible="False" ForeColor="Red">You can't delete sketches that were previously saved.</asp:label></TD>
      <TD vAlign="top" align="right" width="20%"></TD>
   </TR>
   <tr>
      <td vAlign="top" align="right" width="15%"><asp:label id="SketchesLabel" runat="server">Sketches:</asp:label></td>
      <td style="WIDTH: 880px" vAlign="top" align="left">
         <P><INPUT id="sketchFileName" style="WIDTH: 440px; HEIGHT: 22px" type="file" size="54" runat="server">&nbsp;
            <asp:button id="Upload" runat="server" Text="Upload"></asp:button>&nbsp;
            <asp:button id="Draw" runat="server" Text="Draw"></asp:button><BR>
            <asp:label id="MaxFileSizeNote" runat="server">A maximum of 5Mb is allowed for each file.</asp:label><asp:label id="SketchError" runat="server" ForeColor="Red" Visible="False">The sketch was not recognized as a valid image.</asp:label><asp:label id="SketchTooBig" runat="server" ForeColor="Red" Visible="False">The file was too big.</asp:label><asp:label id="SketchNotFound" runat="server" ForeColor="Red" Visible="False">Unable to locate the request address on the map.</asp:label></P>
      </td>
      <td vAlign="top" align="right" width="20%">
         <DIV style="BORDER-RIGHT: black 1px solid; BORDER-TOP: black 1px solid; OVERFLOW: auto; BORDER-LEFT: black 1px solid; WIDTH: 266px; BORDER-BOTTOM: black 1px solid; HEIGHT: 100px"
            align="left"><asp:checkboxlist id="SketchList" runat="server" Width="160px"></asp:checkboxlist></DIV>
         <asp:button id="Delete" runat="server" Text="Delete"></asp:button></td>
   </tr>
   <TR>
      <TD colSpan="3"></TD>
   </TR>
   <TR>
		<TD colspan="3" align="center">
			<iframe id="kMapFrame" style="DISPLAY: none" name="kMapFrame" frameBorder="1" runat="server"
			width="830px" height="630px">
			</iframe>
		</TD>
	</TR>
	<tr>
		<td></td>
	</tr>
	<tr>
		<td colspan="2" align="right">
			<asp:Button id="getImageButton" runat="server" Text="Get image" Visible="False"></asp:Button>
		</td>
	</tr>
	<TR>
		<TD colSpan="3"></TD>
	</TR>
   <tr>
      <td align="right" colSpan="3">&nbsp;
         <asp:button id="Save" runat="server" Text="Save"></asp:button><asp:button id="Cancel" runat="server" Text="Cancel"></asp:button></td>
   </tr>
</table>

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Code behind:

// Set the map viewer url
                  string src = System.Configuration.ConfigurationSettings.AppSettings["MapGuideViewerAddress"];

                  // Set the sketch file name
                  DateTime dtNow = DateTime.Now;
                  string sketchName =  SessionHelper.CurrentTicket + "_" ;
                  sketchName += dtNow.ToString("dd_MM_yyyy_HH_mm_ff");

                  SketchFileName = sketchName + ".jpg";

                  // Add the url LatLong parameters
                  src += Regex.Replace("?lat=" + ptLatLong.Y.ToString() + "&lon=" + ptLatLong.X.ToString(),",",".");

                  // Add the url language parameter
                  src += "&lang=fr" ;

                  getImageButton.Visible = true;

                  // Display the PHP iFrame
                  kMapFrame.Style.Add("display", "");
                  kMapFrame.Attributes.Add("width", "830");
                  kMapFrame.Attributes.Add("height", "630");
                  kMapFrame.Attributes["src"] =  src;

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Thanks!