Upgrade pfSense hanging

I tried to update my pfSense from version 2.4.5 to 2.5.0. But I always got the following message in the GUI.

Please wait while the update system initializes

A reboot of pfSense or cleaning the packages with pkg-static clean -ya was not solving the problem. After a while I found a workaround by disabling IPv6 under System → Advanced → Networking.