Well no thanks to you people ?? I managed to find the solution myself. There is no theme file that needs editing, so you need to add a filter to your functions.php file in the theme directory. Use this
function replace_the_password_form($content) {
global $post;
// if there's a password and it doesn't match the cookie
if ( !empty($post->post_password) && stripslashes($_COOKIE['wp-postpass_'.COOKIEHASH])!=$post->post_password ) {
$output = '
<form action="'.get_option('siteurl').'/wp-pass.php" method="post">
'.__("This post is password protected. To view it please enter your password below:").'
<label for="post_password">Passwort:</label>
<input name="post_password" class="input" type="password" size="20" />
<input type="submit" name="Submit" class="button" value="'.__("Submit").'" />
</form>
';
return $output;
}
else return $content;
}
add_filter('the_content','replace_the_password_form');
Obviously, now you can edit it to your liking.
I found this info on this site:
https://simbo.de/blog/wordpress/wordpress-tipp-passwort-formular-bei-geschuetzten-beitraegen-anpassen/