We need to delete the whole pub/static/frontend-folder if we want to have CSS changes and stuff available on the site.
So we do the following:
$ rm -rf var/cache/ var/generation/ var/page_cache/ pub/static/
$ php-cli bin/magento setup:upgrade
$ php-cli bin/magento setup:static-content:deploy de_DE
Here comes the problem. If you delete the backend folder in pub/static Magento tries to allocate the requested URL (i.e. login page for backend) for the en_US language, even if the only user in the backend has selected that his main language is de_DE.
Magento simply tries to access files that are not there and that are not needed to be compiled (in our case: German). There should be an setting in the backend that sets the main language for the whole store to a specific language which get then called by default. This prevents unnecessary white pages.