Hi,
I've just upgraded our enterprise from 10.5 to 10.6.1.
When I go into the Server manager and try and view the service using ArcGIS Javascript API the window is blank. I can view the service ok through Portal.

I found the following thread that suggest the service directory may be corrupt and have checked our configuration (refer to below).
REST - ArcGIS Javascript Preview not Working after upgrade to 10.4.1
In the developer console I'm getting the following error indicating that these files cant be loaded.

Our IT department have removed internet access for the ArcGIS Server. So I'm assuming that the Server needs internet access to render the previews via arcgis.com? Do I need to allow internet access and add a rule allow traffic to ESRI only *arcgis.com?
Is there an option to use this offline?
Thanks,
Clinton