Your Widget Permissions (Admin > Appearance > Widgets > Permissions for: [Wiki]) had not been converted from the XF1 format by the upgrade due to a bug in the upgrade. This will be fixed for future upgrades from XF1 to XF2 as of the next build; sites that already upgraded would miss out on this fix as you can only upgrade from XF1 to XF2 one time.
However, widgets were working in wiki pages if you manually updated the permissions. I went ahead and converted the XF1 permissions for you to save you time.
In non-wiki pages, widgets were only working if the same widget was selected to appear on the wiki index. This is fixed in the next build; now if a widget is rendered in a non-wiki context, it will assume that it was selected to appear there.