Redirect http to https via .htaccess file

To show customers that they visit a secure site, I bought a SSL certificate and installed it on the server of the host. To redirect from http://www.planet-upcycling.de to https://www.planet-upcycling.de I put the following code into the .htaccess file and stored it into the root folder:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://www.planet-upcycling.de%{www.planet-upcycling.de} [R=301,L]
</IfModule>

Everything seems to work fine for a while, but than I got a 404 mistake on all subpages. Only the start page was visible. When I download the .htaccess file, it contains now the following code:

# 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

What went wrong and what do I have to change? And sorry, I’m not used in working with that kind of codes and htaccess files.