Yes,
I first ran mysql_install_db, then I followed the procedure from the mysql manual for resetting a forgotten password.
What got me was the fact the the root user had a zero length password, as opposed to no password.
You then need to run mysql -u root -p mysql
It will then ask you for a password, and then you press enter. I also recreated the horde database. First I dropped it, and then you run
mysql -u root -p mysql < hordeinstall.sql
I then recreated the hordemgr user, with its password (check their install files), and gave it back its privileges to the database.
HTH,
Alex