Perdonadme, con el lío de vida que llevo olvidé contaros la solución. El problema estaba en el diseño del skin. Os copio la solución que me envió el fabricante:
Cambios para aplicar en cualquier .aspx:
<div class="Container-1">
<div class="containerhead"><span class="dnnicon"><dnn:ICON runat="server" id="dnnICON" /></span><dnn:TITLE runat="server" id="dnnTITLE" Cssclass="title" /><span class="actions"><dnn:ACTIONS runat="server" id="dnnACTIONS" /></span><span class="visibility"></span></div>
<div class="containermain">
<div id="ContentPane" runat="server" class="containerpane"></div>
<div class="clear"></div>
</div>
<div class="containerbot"><span class="left"><dnn:ACTIONBUTTON1 runat="server" id="dnnACTIONBUTTON1" CommandName="AddContent.Action" DisplayIcon="True" DisplayLink="True" /></span><span class="right"><dnn:ACTIONBUTTON4 runat="server" id="dnnACTIONBUTTON4" CommandName="ModuleSettings.Action" DisplayIcon="True" DisplayLink="False" /></span></div>
</div>
<div class="spacer"></div>
Cambiar a:
<dnn:ACTIONS runat="server" ID="dnnACTIONS" ProviderName="DNNMenuNavigationProvider" ExpandDepth="1" PopulateNodesFromClient="True" />
<div class="Container-1">
<div class="containerhead"><span class="dnnicon"><dnn:ICON runat="server" id="dnnICON" /></span><dnn:TITLE runat="server" id="dnnTITLE" Cssclass="title" /></span><span class="visibility"></span></div>
<div class="containermain">
<div id="ContentPane" runat="server" class="containerpane"></div>
<div class="clear"></div>
</div>
<div class="containerbot"><span class="left"><dnn:ACTIONBUTTON1 runat="server" id="dnnACTIONBUTTON1" CommandName="AddContent.Action" DisplayIcon="True" DisplayLink="True" /></span><span class="right"><dnn:ACTIONBUTTON4 runat="server" id="dnnACTIONBUTTON4" CommandName="ModuleSettings.Action" DisplayIcon="True" DisplayLink="False" /></span></div>
</div>
<div class="spacer"></div>
O cambiar a:
<div class="Container-1">
<div class="containerhead"><span class="dnnicon"><dnn:ICON runat="server" id="dnnICON" /></span><dnn:TITLE runat="server" id="dnnTITLE" Cssclass="title" /><span class="actions"><dnn:ACTIONS runat="server" id="dnnACTIONS" ProviderName="DNNMenuNavigationProvider" ExpandDepth="1" PopulateNodesFromClient="True" /></span><span class="visibility"></span></div>
<div class="containermain">
<div id="ContentPane" runat="server" class="containerpane"></div>
<div class="clear"></div>
</div>
<div class="containerbot"><span class="left"><dnn:ACTIONBUTTON1 runat="server" id="dnnACTIONBUTTON1" CommandName="AddContent.Action" DisplayIcon="True" DisplayLink="True" /></span><span class="right"><dnn:ACTIONBUTTON4 runat="server" id="dnnACTIONBUTTON4" CommandName="ModuleSettings.Action" DisplayIcon="True" DisplayLink="False" /></span></div>
</div>
<div class="spacer"></div>