Then I repeated the configuration via network cables and without the wizard. In the first step should exclusively be a connection between PC and WA750RE (connection to the router disconnect before!). Then the WA750RE via DHCP assigns an IP address, and you can configure it.
In my experience, I recommend insistently to turn off the default "Smart IP" mode, assign a static IP address to the device to disable (in the subnet of the router, but outside of the DHCP pool) and the DHCP server of WA750RE. After that is still the WLAN built from the device to configure as well as a profile for the used "source-Fi" set.
Now the unit is functional and can also be weiterkonfiguriert via WLAN (I recommend to change the access to the web interface). The circular arrangement of the status LEDs (all same color), I find highly idiotic. A reading is possible only by very close examination.
The actual function as a repeater (Range etc.) has proved to be with me as well. The additional use as a wireless client for a wired LAN device (which for me was an important selection criterion) also works perfectly.