WL 8.5.20 on Yosemite

Tried to install WL 8.5.20 on a iMac Retina with Yosemite:

Loaded WaveLab_8.5_Installer_mac.dmg

But get an error message: Image not recognized

see Screenshot

See here http://www.steinberg.net/forums/viewtopic.php?f=189&t=65342 for that problem.