Hi
I have never tried using the ASPxUploadControl from DevExpress so can not be of direct help.
However... I mostly use Telerik Web Controls with mojoPortal and seem to remember that when I had trouble with their Upload Control not working. The solution to getting mine to work was...
In the mojoPortal skin I was using, I had to replace the standard ScriptManager and use the Telerik ScriptManager.
In the skin Layout.Master file Replace
<asp:ScriptManager ID="ScriptManager1"..................
With
<telerik:RadScriptManager ID="RadScriptManager1"..................
That's my hunch. Hope it works.
Rick