Hello Markus,
I hope you noticed in Bareos documentation, that connection between php-fpm (webui) and the director had to be unencrypted (due to missing tls-psk support in upstream php-curl module).
But to have https and as such tls connection between your browser and the webui you will have to find how to activate ssl/tls module for your favorite httpd server on your distribution.
Globally it means activate the encryption module on the webserver and install a certificate.
Hope this will drive you to a solution.