If you get this error, you've most likely done a file-based MySQL backup restore, and the InnoDB files are hosed. The horde_sessionhandler table isn't a MyISAM table at all - it's actually an InnoDB table. The easiest way to fix the issue is to stop MySQL and trash the .frm:
# /etc/init.d/mysqld stop
# rm /var/lib/mysql/horde/horde_sessionhandler.frm
Now start MySQL and re-create the table:
# /etc/init.d/mysqld start
# mysql -u admin -p`cat /etc/psa/.psa.shadow`
Here's the SQL statements to run:
CREATE TABLE horde_sessionhandler (session_id VARCHAR(32) NOT NULL, session_lastmodified INT NOT NULL, session_data LONGBLOB, PRIMARY KEY (session_id)) ENGINE = InnoDB;
GRANT SELECT, INSERT, UPDATE, DELETE ON horde_sessionhandler TO horde@localhost;
You're good to go!
