Please add the line "RewriteRule /SOGo(.*) https://%{HTTP_HOST}%{REQUEST_URI}" in sogo-domain-de.conf too.
Since we don't hard-code the server name or url in "x-webobjects-server-name" and "x-webobjects-server-url", SOGo will always tries to access current http host name in URL (the one in your web browser, https://<host>/SOGo), so you should make the SOGo http access redirected to https.