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If you had written a custom template based on the default Gantry template (rt_gantry) , the updated Gantry library Andy mentioned might not still fix the issue. If after the update your admin is still not allowing changes, please edit the template-options.xml found in your custom template and look for the entry:
<fields name="menu" type="selectedset" label="MENU_CTRL">
<field name="enabled" type="toggle" default="1" label="SHOW" />
and change it to
<fields name="menu" type="selectedset" label="MENU_CTRL">
<field name="enabled" type="toggle" default="1" label="SHOW" enabler="true" />
This fix has been applied already on the latest version of Gantry.
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Djamil Legato
Turin, Italy
Rockettheme, LLC
Saluti
Davide