Ok. I think I finally found the root cause. It also fixed issues with backing up the configuration:
- In Safari, go to Preferences -> Security -> "Website Settings" (next to "Plug-Ins") -> Select Java on the left -> on the right, you will see a list of web sites that you trust. For the IP address of your ISY device, change "Allow" to "Run in Unsafe Mode".
Once you have done it, open the admin app again and it should work.