Since we spent days (let me write it again, daaayyyyyyssss) to solve this problem. To prevent you from doing the same, I write this entry.
Our problem was, that a guest box was running fine on a virtual box host. The "only" issue was, that everyone could ping (and even do more) with the host and the guest os, except the host talking with the guest and vice versa.
It looks like this is a microsoft windows 10 issue only. Everything is workin on my *nix systems - meaning, if you are using real software, no problem is there.
But seriously folks (I know you are not reading this entry because you want to have some funny battle from good, free as in freedom software, against clunky ugly and bad close source whatever), the solving of this issue is braindead simple.
It a also contains a workaround: - Open the network adapters in Windows 10 - Open properties of the adapter used in your guest OS - Disable IPv6 -> OK - Open properties again - Enable IPv6 -> OK