@Sigfrid_Xanten 

Here are two things you could try that might help with troubleshooting the problem.

1) Connect your Android phone to the Wi-Fi of your Archer C7, open the "OpenVPN Connect" app and then activate the connection that you've already imported ealier. 
Does it connect successfully? (normally it should connect)

2) Enable "Remote Management" on the C7 via the menu "Advanced -> Administration". 
Then grab a device that is connected to the Internet via a different provider (like a phone with its own SIM card), open the device's web browser and type http://78.132.82.195 (or http://vifacloud2020.ddns.net). 
Does the login page of your Archer C7 appear in the browser?
 

  @woozle 

Thx for the first ideas.

Ad 1) Nope. Neither in WiFi nor in mobile Internet I get a connection in the openvpn app on Android. I click on the slider but it does not start (or it seems to stop immediatly, in comparison to the Windows Client which tries to connect for a minute or so)

Ad 2) Yes, I can enter and administrate the C7 via mobile Internet through both the IP as well as vifacloud2020.ddns.net

Further ideas? Thank you so much.

By the way: I also added and activated Port 1194 under NAT/PORT TRIGGERING. Did not help.

  @Sigfrid_Xanten 

According to the log you provided earlier there is no response from the VPN server. (Server poll timeout, trying next remote entry...)

I can only think of two possible reasons. 

1) The incoming connection from the Internet does not go through to the router.

-> We can now rule this out, because you can get to the remote login screen of the C7 via the Internet.

2) The openvpn server process does not start because something got misconfigured internally and disabling/enabling the OpenVPN option via the router's admin webpage doesn't correct the error.

-> Since the C7 (as well as most other commercial routers) doesn't provide access to the Linux command line we cannot verify this.
The only way to recover from such a situation would be to reset the router to factory default settings.

By the way, in case you decided to go for the factory restore, then test the OpenVPN server first thing after the reset. What I mean is, don't change a whole lot of other settings on the router before you test the OpenVPN.

After the router reset a new certificate will be created, which means you have to export the ovpn profile again and import it to your client devices.

And don't add port triggering rules for ports used by the C7's own VPN service.