1. In WHM, I did SQL Services -> Repair a Database, and chose "horde" database.
2. then went to WHM -> Restart Services -> SQL Server (MySQL) to perform a MySQL Service restart.
This did the trick and I was able to login again.
I did NOT have to do the /usr/local/cpanel/bin/update-horde --force, nor was skip innodb in my /etc/my.cnf file.
The Repair Database listed everything as OK, except something about a session handler process. So I suspect the database was fine, and it was simply the MySQL restart that fixed it.