Q: Two finger scroll no longer works

Upgraded to Sierra yesterday. Two-finger scrolling no longer works. Called AppleCare. They suggested I re-install. That didn't fix it. (Also previously zapped the P-RAM... and did one other restart with certain keys pressed.)

The next option that was recommended was to delete my preferences folder. That will lose ALL my sign-ins and other preferences so I am not ready to do that yet.

Haven't seen any other examples of this happening (except one here that mentions three finger drag...)

Any ideas?

Thanks.