@mmaunder,
After I read your answer, I thought this problem is related to user on my web server.
I changed my website's owner permissions from "user:www-data" to "www-data:www-data"
And now, It worked for me.
cd /var/www/mywebsite.com/
find -exec chown -R www-data:www-data {} \; -print
find -type d -exec chmod 755 {} \; -print
find -type f -exec chmod 644 {} \; -print
chmod 600 wp-config.php
chmod 604 .htaccess