I know you tried using a skin that came with mojo to rule out the ScriptLoader issue but site skins are not updated when you run a mojoPortal update. The reason for this is to prevent the loss of changes you may have made to the skins since installation. To update the skins to their newest versions, you will need to copy the updated skins from \data\skins (or a zip of the latest release) to \data\sites\[sitenumber]\skins. Be careful when doing this as any changes you have made to the skins that come with mojoPortal will be overwritten. As a side note, I always rename skins before modifying them so I don't have to worry about losing my changes if I want to copy the skins from a release to my site(s).
The biggie here is that you must have the ScriptLoader so I would check for the line below in your current skin or in the skin you set on your test page.
<portal:ScriptLoader id="ScriptLoader1" runat="server" />
HTH,
Joe D.