Airport Express - Connection lost & weird solution

Hi - my MacBook Pro Retina 15'' (2012) looses it's connection to the internet every few hours. WIFI connection is still there and strong, I just have no access to the internet.

My setup: Airport Express is connected to the router I got from my ISP with an ethernet cable. Airport Express is not extending a WIFI network, the router is not creating a WIFI network on it's own.

I'm not experiencing these issues with any other device (iPad, iPhone, MacBook Air, Android Phone...) only with the 15'' BUT on another location I'm using a very similar setup with an Airport Express an it works flawlessly with the 15''.

Now the weird thing:

The only way to fix it is to open the AirPort Utility App on my Mac. As soon as I see the interface of the App the internet connection is back instantly.

This is no coincident, it works every time.

I've tried to reset the Airport Express and created a new network with a different name. Problem is still there.

Thanks!