I've finally found an answer:
MacBook Pro experiencing ping spikes to local router
I've done the following:
Enable Wi-Fi logging
Perform ping: ping 126.96.36.199 -i 0.1
Check for changes on wifi log after ping spike: tail -f /var/log/wifi.log
airportd Successful cache-assisted scan request for locationd
airportd Successful cache-assisted ...
in the meantime problem is solved. I don't know what solved it. But I let it continue the 2nd time. After 30 min it ended up with an installation error, mentioning something about memory access problems. I restarted the whole thing again. Everything was the same, but this time it worked :). Now the next part, where it all started. Install windows 7 on ...