I came across this error yesterday for the first time in Plesk 8.2.1. Following solution corrected the error
1. Log in to the Mysql shell as root or open phpmyadmin as admin/root user.
2. Issue the following command in the Mysql shell
UPDATE psa.hosting SET webstat = 'none' WHERE webstat = '';
If you use phpmyadmin invoke the command without the “;” symbol at the end.