The {0} account must have a system user name because the database is set to use system user names.
Explanation
When the database is set to use system user names, all users must have system user names. The database is set to use system user names when the "Use system user names" option is enabled in the database.
User response
Click Tools > Manage Users. Select a user and click Edit. On the Details tab, type a system user name for the user.