Now, if you had the old Atahualpa 2.x version on the web server (which lives in /atahualpa/) and uploaded 3.1.9 from wordpress.org (which is also in /atahualpa/) then you might very well have both Atahualpa 2.x files and Atahualpa 3.x files in the same directory /wp-content/themes/atahualpa/ on the web server.
Even if your FTP program overwrote existing files, errors still may happen because Atahualpa 2 had files that Atahualpa 3 does not have anymore, such as "archive.php".
When serving an archive page Wordpress will look for "archive.php" first, and since you may have an old "archive.php" laying around in the theme directory (from an old Atahualpa 2 version), Wordpress will happily serve that archive.php (from Atahualpa 2) instead of index.php (from Atahualpa 3)
If you downloaded a Atahualpa 3 version and the folder name is /atahualpa/ instead of /atahualpa3/, and you already have Atahualpa 2 installed on your WP installation:
- change the folder name to /atahualpa3/ on your desktop computer before uploading (recommended) OR
- delete a possibly existing /atahualpa/ (from Atahualpa 2) on the web server before uploading the new /atahualpa/ (as Atahualpa 3)