Can't flash freecyngn after installing CM 12.1 on my Z3 Compact

As the title says, I'm trying to install freecyngn from MaR-V-iN but keep getting the following error:

Install failed
finding update package…
openeing update package…
verifying update package…
E: footer is wrong
E: signature verification failed
Installation aborted

I followed the instructions in the official thread/readme carefully, flashed the latest 12.1 CM NightlyBuild on my Z3 Compact, didn't reboot, tried to flash freecyngn-flashable-cm12.zip but without luck :/
I have the feeling this new, downgraded CM-Recovery is to blame, cause the instructions speaks of CWM-Recovery.

Ideas?

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fuggat111