Error Code=100

Try logging out and in again. The security code ( nonce ) is likely to be outdated.

Can you supply a link to a page with a slideshow, and male sure one can vote when logged out, so i can try it myself.
I do not read greek, you know, so i can not navigate on the site.

You have ratings, so it worked in the past.
I see no strange things in the wppa generated html.

I suspect a conflict with another plugin, especially any plugin that speeds up the page by any means. Did you recently add such a plugin or changed settings of such a plugin?

Can you try eliminating the above by temporary de-activatig All in One SEO Pack and Google Analytics by Yoast and any other plugin that you may have added lately?

I want to insert a diagnostic message.

Either give me administrator rights, or – if you are capable of this – edit wppa-ajax.php around line 476 it reads:

// Make errortext
$errtxt = __( 'An error occurred while processing you rating request.' , 'wp-photo-album-plus');
$errtxt .= "\n".__( 'Maybe you opened the page too long ago to recognize you.' , 'wp-photo-album-plus');
$errtxt .= "\n".__( 'You may refresh the page and try again.' , 'wp-photo-album-plus');
$wartxt = __( 'Althoug an error occurred while processing your rating, your vote has been registered.' , 'wp-photo-album-plus');
$wartxt .= "\n".__( 'However, this may not be reflected in the current pageview' , 'wp-photo-album-plus');

change to:

// Make errortext
$errtxt = __( 'An error occurred while processing you rating request.' , 'wp-photo-album-plus');
$errtxt .= "\n".__( 'Maybe you opened the page too long ago to recognize you.' , 'wp-photo-album-plus');
$errtxt .= "\n".__( 'You may refresh the page and try again.' , 'wp-photo-album-plus');
$errtxt .= "\n\n".'Diagnostics: '."\n".serialize($_GET)."\n".serialize($_POST);
$wartxt = __( 'Althoug an error occurred while processing your rating, your vote has been registered.' , 'wp-photo-album-plus');
$wartxt .= "\n".__( 'However, this may not be reflected in the current pageview' , 'wp-photo-album-plus');

I.e. insert line:

$errtxt .= "\n\n".'Diagnostics: '."\n".serialize($_GET)."\n".serialize($_POST);

I found out.

Somebody has changed the theme, there is a part missing on every page.
I installd atahualpa on my dev site and it works.
On your site a part starting with:

<!-- Footer -->

is missing. In that area the security code that is missing should be located.
Did anybody remove wp_footer() from a template?

Please test on a different theme, or – if you are sure the theme is not modified – de-activate other plugins.