I should have searched better. There's a post on the forum that says the same basic problem occurred when upgrading to Avast 2914.9.0.2021. It said to do a full uninstall of Avast and then reinstall.
Presto! It worked.
When I went back into Outlook, the add in was still disabled (I guess because that was the state when I uninstalled). However, unlike before when enabling caused Outlook to crash, this time, the add in got enabled.