Hi @Berth68
How odd! I have done some further testing to make sure that it worked in different configurations. I tested this with an ArcGIS Online organisation account, a Developer account and a Public account, all of which worked without issue, product the images below:
My guess would be your machines firewall or antivirus that might be stopping the connection to the service?
I would test turning them off temporarily and attempting adding it to see if that is causing the issue.
~Just trying to fix things~