My system had just upgraded to mysql 5.1. I hadn't intended to upgrade. It was just one of those things that happens when you are trying to upgrade one or two packages on a Debian unstable system and find out that everything breaks.

The end result of this breakage was that mysql was uninstalled.

I needed to reinstall it:

aptitute install mysql-server mysql-client

It seemed to work. But if I (as 'root') tried to grant privileges to another user I got that 'access denied' error. What was going on?

The answer seems to be that when mysql was unintentionally uninstalled, it left the existing database files in place and when version 5.1 was installed it started accessing these old, slightly incompatible files. Running:

mysql_upgrade -u root -p

fixed the problem.

Then a second problem presented itself. The maintenance user 'debian-sys-maint' couldn't access the database. This is a fairly common and often discussed problem.

Logging into mysql as the root user:

mysql -u root -p mysql

and granting privileges to 'debian-sys-maint':

GRANT ALL PRIVILEGES on *.* to 'debian-sys-maint'@'localhost' IDENTIFIED BY '<password>';

where <password> is the password listed in /etc/mysql/debian.cnf fixed the problem.

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