Posts by author displays error "No posts found"

Hello,

I’m responsible for a small blog running on WP 3.0.5.
The blog was feeded by one main author (editor). Now we got a second author, which I also created as a Editor in the WordPress Admin Panel. They are both displayed with the full name “First name Last name” (i.e. John Miller) on the Blog below the post title. The main reason why I want to show the author name is to display all the posts by the specific author.

If I click on the main author (author #1) all of her posts are shown without any problems. If I click on the second author (there is actually only one post at the moment) the blog post appears but also with an error message appears above: “No posts found…”

If I create a second post (with author #2) the same problem appears… Only the first post of this author will be displayed and the “No posts found” message. The second post is not being displayed.

Then I tried to copy the original “loop.php” file (from the twenty one theme) to my server to overwrite the existing one (which I already edited)… but the problem still appears.

I did search on Google for that kind of problem, but didn’t find any answers.
Hope someone knows this problem and how to solve…

Thanks a lot (in advance),
Michael