Installing WordPress into a subdirectory today produced an error “Please provide a valid username” (wp-admin/install?step=2).
No matter what I tried, WordPress wouldn’t accept the username, even though it fulfilled the validation requirements.
It turns out that if you have a .htaccess file in the root of your website your username won’t be accepted. Temporarily rename .htaccess and your installation should be accepted. (Don’t forget to rename it again!)