sliderbl on "Failed to upgrade to WP 4.5, resulting a 500 server error"

Thank you very much for your reply.
My Wordpress is located at the htdocs/ file, and now its htaccess is:

php_value display_errors Off
php_flag magic_quotes 1
php_flag magic_quotes_gpc 1
php_value mbstring.http_input auto
php_value date.timezone America/New_York

I found the backup two months ago, the htaccess is written like this:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

And I see a htaccess before the htdocs folder (i.e. in the index):

ErrorDocument 404 /notfound.html
ErrorDocument 403 /notfound.html
ErrorDocument 500 /notfound.html

Though I am not sure if this one matters. And the last modified date was in 2014.

Thank you.