3.1 stable IPs of a client can not be selected

System is a Debian 8 with Apache, PHP 5.6 and ISPConfig 3.1, updated from latest stable before 3.1.

My server IPs do belong to my client (C1):

If I create a new website in ISPConfig, I can select all of the IPs:

But if I select the client C1 - which has all these IPs assigned - I can no longer choose the IPs, the list is empty:

This worked perfectly before updating to 3.1.

Existing websites do have the client and the IPs set just fine.

Any support would be great.