OpenVPN webgui configuration fails on ARM

Also on the LAN at home.
I only want to secure the WLAN communication between the RPis and the OMV so no one can hijack a connection or sniff on the data.

Äh, where would you have expected it ?

If you have this all on same subnet behind your router what are you worried about??? You do not need VPN for this. Just make sure that the ports you use to communicate to the RPis are closed on the router's firewall. What service, program, are you using to control the the shades in the home??? If you access this service manully from WLAN use OpenVPN AS to connect to your OMV. Then access the service for manual control.

ok, but you don't see a point in sniffing data from the WLAN and hijacking a connection.
Hm. ok.

But is there a manual how and what to set up on OpenVPN, in case I intent to connect it to my OpenVPN sandbox on a cloud system?

Is there a chance to assign the transfer network adresses depending on the client who connects ?
What is Client-to-client especially for ?
Where do I find further infos on what the OpenVPN server on OMV configures exactly ?

Thanks

There is nothing to worry about if the ports used between the Rpis and OMV are not open on your router's firewall.

OpenVPN is to connect you to a network (your home) when you are in a remote (away from home) loctation.

For OpenVPN AS there is information TAB next to the Settings TAB when in the plugin. It tells you how to setup the server.

I have not done a info. page for the OpenVPN. But I have helped plenty of people. Here is a thread that has information on Setting up that server:
Setting up openvpn
See my 3rd, 4th and 5th posts. The 4th post tells how to setup the client for the OpenVPN Plugin.

Basically, a VPN will connect you to your LAN. It does not assign you an IP address on your LAN though. It creates it's own subnet and you are assigned an ip on that subnet (virtual network). That address on the virtual network is allowed to communnicate with devices on your LAN's main subnet (usually 192.168.1.x).

The best way to start with this is to setup one server and one client. Ask questions are you go and provide screenshots of your settings for the server. With the client just say what type of client machine you are using and then instructions can be provided.

I always setup a server first and then the client on my Samsung (Android OS) cell phone. You have to be out of your LAN to test the VPN server. That is easiest on your Cell (Handi) with the OpenVPN Connect Client app.... You disconnect your wifi connection. You get remote ip from phone network via cell tower.