[charlesr]

There is probably someone who knows what logs to look in to see the problem, but that's not me.... Meanwhile:

Is the user table ok? Try repairing it. You may need to stop mysql first for a successful repair on the user table:

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To repair all the tables in the database in centos we stopped it first.
# /etc/init.d/mysqld stop
Then navigated to the database files:
# cd /var/lib/mysql/[databasename]/
Then run the following command:
# myisamchk -r *.MYI
And once complete restart MySQL:
# /etc/init.d/mysqld start
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If it's ok, then do you have a busy forum / is your server powerful enough? I had some odd behaviour until I upgraded and then everything magically worked perfectly.