Yes, Grafana server and VPN (server? I think it's called client..confused) running on the same host.
I'm able to connect to any devices which are hosted on the same machine. There is another RasPi in the same LAN, which I cannot reach via VPN. But I think that is normal unless I set up bridge mode at the host running VPN.

Here are the firewall rules:

$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination        

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination        
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     all  --  10.8.0.0/24          anywhere            
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     all  --  anywhere             anywhere

$ sudo iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination        
DNAT       tcp  --  anywhere             10.147.17.36         tcp dpt:4000 to:192.168.178.36

Chain INPUT (policy ACCEPT)
target     prot opt source               destination        

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination        

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination        
MASQUERADE  all  --  10.8.0.0/24          anywhere            
MASQUERADE  all  --  anywhere             anywhere

Yes I too think it has to do with networking rather that a grafana problem. But as said in my initial post, I'm a complete networking noob.

---