The ISP is Unity Media (Germany). I contacted them in the first day when I found the problem. Because they give only IPv6 to the new clients there is no port forwarding anymore. They can’t help. So I am trying to find out is the router what blocks the Upload from the cloud (Remind you I am only not able to download from it, anything else is possible. Where uploading to it is also slow but at least works), or the ISP blocks somehow that kind of connection. Or the WD servers (for the relay connection) have only IPv4 and there is actually not full connection to the cloud which has IPv6 and the IPv4 which I have as a client to be able to use internet normal is a server from the ISP site.
IPv6 - Client IPv6 able to connect to IPv6 servers directly. No need of forward.
ISP <
IPv4 - Client has only IPv6. If Client want to open/connect/communicate with an IPv4 server the packets go thru the ISP IPv4 server, then that server contacts the server which the client wants to connect. That server answers to the ISP IPv4 server and that ISP IPv4 gives the packets to the client (IPv4 Tunnel)… No way to forward beacuse it is ISP controlled.
I don’t know how to explain it 
