Make sure that there are no records with empty 'user' or 'password' fields in 'user' table in 'mysql' database. It can be found using the following command:
MYSQL_PWD=`cat /etc/psa/.psa.shadow` mysql -u admin -D mysql -e 'select * from user where user="" or password=""'
The output should be empty. If not, delete records with empty 'user' or 'password' fields.