Thanks for that idea!
I will look into that during the next week or maybe the next weekend.
For now I have found following workaround:
I started the installation with the august version. After the reboot I updated the local databases and then installed the new welcome. I used the update mirror to get the fastest local mirrors. At the end I made a complete system update and installed gnome and gnome-extras. Now I have a completely updated system and Gnome is working fine.