This happens, it can be fixed simply. In the webui where you create the address book, look to the dropdown on the far right.
Use it to Export the address book and save it to your desktop (Don’t check the “Legacy mode” option)
Then use it to Delete All
Then use it to Import from file from your desktop (Choose option 2 if prompted)
It will now allow you to populate the new user(s)
There is a bug that causes the issue, it has affected every firmware level and this is the only known workaround, luckily it works in every case and is simple to do. The machine doe snot need to be reloaded or rebooted, just 3 steps that take under 2 minutes.