I can recommend Uberspace especially if you understand some German (the registration process right now comes in German although Google translate should be able to help). The documentation is available in English though.
Uberspace right now only offers shared hosting but comes with a lot of freedom for those who are willing to work with a command line interface (no cPanel, Confixx or Plesk). Installing Flarum was easy, composer is installed by default, HTTPS is enforced by default, you don't need to care about that.
You decide how much you want to pay, but minimum price should be 5 Euro per month. If your price doesn't match your strain on the server they will tell you. All in all very nice people and a professional approach.