Very important after upgrading:
Generally, having successfully updated/upgraded the Gantt-Chart add-on, please click on "Gantt-Chart" menu item on the left side within section "time tracking" on page "manage add-ons" as system administrator. You can just take over the default settings or modify them according to your needs. In both cases, you MUST click on the "apply" button at the bottom. Otherwise, nothing will be done and not even (new) defaults are stored!