Hi Tobias, welcome into our community!
In order to communicate an array element, first of all you must enable “show array elements” for this variable in server OpcUa mapping configuration.
Afterwards, if you use Ua Expert software you will see how inside variable (in this case varArray), there are more variables. A variable for each array position.
Then, if you follow the post How to communicate two controllers thought OPC UA , you have just to go “opening folders” and adding the path accordingly with its NamespaceIndex. In this case:
/0:Root/0:Objects/4:PLC/6:Modules/6:&:&:/6:Program1/6:varArray/6:varArray[0]
And at this point you can test it. And it works 
Hope this solves your problem!