It's now the third customer telling us that his AGS services are "empty", showing null extents and no layers/properties for MapServers. All have ArcGIS Enterprise 10.6 installed.
The only thing we noticed is that all three customers servers did a Windows update, and after that this happened.
Did anyone noticed the same behaviour? Unfortunately I cannot really tell now what OS the servers are, except one which is Windows Server 2016 Standard.