@stefanot,

I would be inclined to first believe that this may be a result of the quality of your Powerline connection. Homeplug Powerline is always finicky and is far more susceptible to interference from appliances and power fluctuations.

Due to the additional hop they must complete, VPN Connections often have "higher" requirements to maintain an active connection. For example, if your PC needs to send data to maintain the connection every 30 seconds, one blip in the powerline network could cause the drop if it happens at the wrong time. With a normal connection, your PC will simply resend the request a few times over and doesn't rely on the server having an active connection through the VPN to send the request.

If your Powerline network is crossing circuits or is running a long way, you may see these types of behaviors. I would recommend, if you can, moving all the adapters and PC to the room with your router to see if the behavior persists when the adapters are located in the same room/circuit.

You may also be able to access the system logs of your Powerline connectors through the WebUi while using the VPN, this may give a better indicator of whether it is being caused by the physical link or as a result of the software/routing.