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 Maintenance update 8.5.20? - WaveLab - Steinberg Forums for that problem.