Post's author link not working a bit …

This is the link at the top of a post whos anchor text is the name of the author and redriects to a page that should list all posts by that author.

I’ve got two users. It works for one (username: Sandy, nickname: Sandy) and not for the other (username: wbcadmin, nickname: Website Administrator)

The author link points to:
domain.com/author/WBC%20Administrator/

which shows the standard “not found” page, as does:
domain.com/author/wbcadmin

Any clues why this isn’t working for the admin account?

Cheers

David