Telerik RadGrid not opening popup editor - telerik

Question

I've a Telerik Radgrid displaying a list of customers. In the grid, I have a command button that's supposed to open a popup editor to add new customers. For some reason, the popup does not open. It shows a small flicker and nothing happens. Strangely enough, if I go into the database, manually add a customer record and refresh the grid, I'm able to open the editor.
Here is the grid definition in my ascx:
<telerik:RadGrid ID="CustomerList" runat="server"
GroupingEnabled="False" ShowStatusBar="True" OnPreRender="CustomerList_PreRender"
DataSourceID="CustomerDataSource"
OnItemCommand="CustomerList_ItemCommand" OnItemDataBound="CustomerList_ItemDataBound"
OnInsertCommand="CustomerList_InsertCommand" OnItemInserted="CustomerList_ItemInserted"
OnItemEvent="CustomerList_ItemEvent" >
<MasterTableView DataKeyNames="CustomerId" DataSourceID="CustomerDataSource"
AllowPaging="true" AutoGenerateColumns="False"
GridLines="None" EditMode="PopUp" ShowFooter="True"
InsertItemDisplay="Bottom" CommandItemDisplay="TopAndBottom" >
<CommandItemSettings AddNewRecordText="Add Customer" />
<Columns>
<telerik:GridBoundColumn DataField="CustomerId" DataType="System.Int32" FilterControlAltText="Filter CustomerId column" HeaderText="CustomerId" ReadOnly="True" SortExpression="CustomerId" UniqueName="CustomerId">
<ColumnValidationSettings>
<ModelErrorMessage Text="" />
</ColumnValidationSettings>
</telerik:GridBoundColumn>
<telerik:GridBoundColumn DataField="CustomerName" FilterControlAltText="Filter CustomerName column" HeaderText="CustomerName" SortExpression="CustomerName" UniqueName="CustomerName">
<ColumnValidationSettings>
<ModelErrorMessage Text="" />
</ColumnValidationSettings>
</telerik:GridBoundColumn>
<telerik:GridEditCommandColumn UniqueName="EditCommandColumn">
<ItemStyle Width="20px" Font-Underline="true" />
</telerik:GridEditCommandColumn>
</Columns>
<NoRecordsTemplate>
No customers have been created yet.
</NoRecordsTemplate>
<EditFormSettings EditFormType="WebUserControl" CaptionDataField="CustomerName"
UserControlName="~/UserControls/CustomerEditor.ascx"
>
<EditColumn UniqueName="EditCommandColumn" HeaderButtonType="TextButton">
</EditColumn>
<PopUpSettings ScrollBars="None" Width="581"/>
</EditFormSettings>
<PagerStyle Mode="NextPrevAndNumeric" Position="Bottom" />
</MasterTableView>
</telerik:RadGrid>
Any help is most welcome!