The best way to install FreedomBox on Virtualbox is by using AMD64 version of Virtualbox images. More information is available on the Virtualbox page. This is true even if you are running an Intel machine (amd64 is just the architecture name given to all 64bit Intel/AMD machines). The i386 images were for 32-bit Intel/AMD machines (which are very rare these days). Debian dropped support and we did too.
Installing using “apt install freedombox” or using the Debian installer is fine as well with some differences noted here.
Thanks a lot @joseph and @Sunil ! We will move on tomorrow and see, which way is the best. I did do some test setups with debian (minimal installation without desktop) and freedombox on top of it, which works good. Tomorrow a friend of mine and me will try to install the AMD64 version on his hardware.