@CoLeEe 

Hi,

Is the Powerline connection of the adapters in question strong or rather marginal? 
If you could report us the "Powerline Rates" shown in the tpPLC Utility by the arrows, then we may get an idea how good the connections are.

If the Powerline connections are already marginal under normal conditions, then someone plugging in (or switching on) another electrical appliance connected to the same circuit could degrade the Powerline signal to a point that the connection is lost.

And when I write "electrical appliance" I mean anything that can be plugged into a wall socket. Even someone plugging in a phone charger could potentially bring down the Powerline connection if it was already marginal to begin with.

  @woozle 

Hello Woozle,

Thanks for your fast answer.

Here you can see the screenshot from my application.
 

  @CoLeEe 

Ok. I think these numbers look quite good for AV600-grade Powerline adapters.

Anyway, the issue you've described will be difficult to troubleshoot. 
In case you happened to withness such an interruption of the powerline connection live, then you could ask the other persons in your home if the have operated a certain electric device at the moment the interruption occurred. 
But that's pretty much all that can be done for troubleshooting.

Also, it's probably extremely rare for users to constantly monitor their Powerline connections. So, if this was something that all Powerline adapters do every once in a while, then we might not actually hear about it.

I certainly wouldn't notice if my Powerline adapters disconnected for a period of less than 20 seconds once a day.