Upgraded to 2.9 and widgets on the widgets' page are unable to be dragged. Checked this topic here and found same issue with 2.8. For me, it was the GD Star Rating plugin that needed to be deactivated. Thankfully, this time i didn't have to do the whole deactivate all and reactivate one-by-one.
Checked their site and see that they have a newer version planned that MIGHT resolve this. C'mon ... REALLY? :)