I just discovered a weird problem in my WP. The author archive (author.php) returns 404 error ONLY if i use custom permalink structure like /%postname%/ it shows up fine when I use the Default permalinks. What could be causing that please?
It works just fine for me. Do you have a post also named author or something? You may have some conditional statement somewhere that is dependent on the working permalink structure.
Check to make sure your nickname has no spaces. To be very sure of this you could dig into yout WP database and look for
user_nicename and remove any spaces from those values. Just a thought.