Asp.net controls after UpdateProgress - sometimes works, sometimes not

Hi there,

I meet the issue that  Asp.net controls which should show after the updateprogress finished are sometimes shown, some times not!
Asp.net 4.5.1 / c# / IIS 7.5 / IE10

Flow:

- The user is filling a form and submitting the data via a lablebutton.
- An UpdateProgress is shown
-- Some c# code is launched to handle things in backround
-- A logfile is updated
- A table within a (<p> tag, which is hidden on startup) displays the result of the c# operations ( -- that does not work always -- )

Frontend:

  ...<p></p><h2>Last step:</h2><p></p><p>                 <span class="labelcnt"><asp:LinkButton ID="lbtnCreateUser" runat="server" ClientIDMode="Static" EnableViewState="False" CausesValidation="True" OnClick="lbtnCreateUser_Click">&rsaquo;&rsaquo;&nbsp;Create User.</asp:LinkButton></span>                               </p>           <p runat="server" id="AppFeedbackUI" visible="false"> <h2>Result:</h2><br /><asp:Label ID="lblAppFeedback" runat="server"></asp:Label>                                        <span class="labelcnt" title="Click to emtpy the fields and create a new user."><asp:LinkButton ID="lbtnRestartApp" runat="server" ClientIDMode="Static" EnableViewState="False" CausesValidation="false" OnClick="lbtnRestartApp_Click">&rsaquo;&rsaquo;&nbsp;Restart.</asp:LinkButton></span>                        </p></ContentTemplate></asp:UpdatePanel><asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanelusr"><ProgressTemplate><div class="PleaseWait">
                Working, one moment ...</div></ProgressTemplate></asp:UpdateProgress>      </asp:Content>

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Update on 2015-01-23 (11:00 AM, GMT+8)

As testing purpose I have replaced the

<p runat="server" id="AppFeedbackUI" visible="false"> 

against

<asp:Panel runat="server" id="AppFeedbackUI" visible="false" ClientIDMode="Static" EnableViewState="False">

The result is the same - sometimes it works, sometimes not.
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Backend:

            UpdateLog(adobj, retvalad, objContentMsgad);                      
            AppFeedbackUI.Visible = true;
            string appfeedback = "";

            foreach (var pair in AppFeedback)                            
                appfeedback += "<tr><td>" + pair.Key + "</td><td>" + pair.Value + "</td></tr>";

            string tblcss = "rslttablegood";
            if (appfeedback.Contains("not") || appfeedback.Contains("Not"))            
                tblcss = "rslttablesoso";
            if (appfeedback.Contains("xception") || appfeedback.Contains("code line"))            
                tblcss = "rslttablebad";                            
            lblAppFeedback.Text = "<table class=" + tblcss + ">" + appfeedback + "</table>";         

Many thanks for your suggestions!!

Ruben