STEPS TO REPRODUCE
- Create an instance of XWiki 12.6.4
- Create a dashboard page and edit a gadget on the dashboard
- Upgrade the instance to XWiki 12.6.6
- After upgrade, go to the respective dashboard page
- Edit the dashboard and click the pencil on the top right corner of one of the gadgets
The gadget edit popup displays properly and allows to edit the parameters of the respective gadget.
The edit box is not displayed, the pencil button becomes a spinning wheel that keeps on loading and then disappears.