Thank you for the hint. I first tried your changes but it did not work in Remote Upgrade mode (i did not use an extra answer file, I just used the same commands I posted above).
On second try I used these modifications and now it works!
I unpacked the install.img, modified the script, repacked install.img and placed it on our web server.
My second test system is with X5670, but I also may have some other servers with E5520
EDIT: Remote Upgrade (with patch) worked on
Asrock FM2A88x Extreme4+ with AMD A8-6500
HP ML350 G6 with two Intel Xeon X5670