UpdatePanel inside a bootstrap modal doesn't update - asp.net

Question

I change the label's text that resides with a UpdatePanel that is within a Bootstrap Modal. When the Modal pops up, the text has not been set (through in debugging I can see it sets the labels text correctly with the error message).
This modal resides and is called from within a UserControl.
What am I missing? Why isn't the UpdatePanel being triggered?
<%--ERROR MODAL--%>
<asp:Panel runat="server" class="modal fade" ID="modalErrorMessage" role="dialog" aria-labelledby="modalErrorMessage" aria-hidden="true">
<div class="modal-dialog">
<asp:UpdatePanel runat="server">
<ContentTemplate>
<div class="modal-content panel-danger">
<div class="modal-header panel-heading"><span class="glyphicon glyphicon-warning-sign" aria-hidden="true"></span> Error!</div>
<div class="modal-body">
<asp:Label runat="server" ID="lblmodalErrorMessage" Text=""></asp:Label>
<br />
<div class="modal-footer margin-top-sm">
<asp:Panel runat="server" ID="Panel2">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
</asp:Panel>
</div>
</div>
</div>
</ContentTemplate>
</asp:UpdatePanel>
</div>
Private Sub RaiseErrorMessageModal(errorMessage As String)
lblmodalErrorMessage.Text = errorMessage
RegisterModalPopup(modalErrorMessage.ClientID)
End Sub
Protected Sub RegisterModalPopup(modalId As String)
Dim script = String.Format("$('#{0}').modal();", modalId)
ScriptManager.RegisterStartupScript(Page, Page.GetType(), modalId,
script, True)
End Sub

Answer 1

You should specify an ID for your UpdatePanel and also set the UpdateMode property to indicate how the update panel should update/refresh its content.
<asp:UpdatePanel runat="server" ID="UpdatePanel123" UpdateMode="Conditional">
In your code where you set the text of the Label you should call the .Update() event of the UpdatePanel:
//Depending on where this code is, you may need to find your UpdatePanel control.
Private Sub RaiseErrorMessageModal(errorMessage As String)
lblmodalErrorMessage.Text = errorMessage
UpdatePanel123.Update()
RegisterModalPopup(modalErrorMessage.ClientID)
End Sub
MSDN reference: https://msdn.microsoft.com/en-us/library/system.web.ui.updatepanel.updatemode(v=vs.110).aspx?cs-save-lang=1&cs-lang=vb#code-snippet-1