As Title says, its staightforward and easy to reproduce.
Reaktor Ensemble is to be saved along with cubase project, with ‘Automatic saving with host’ enabled.
With this option enabled, if I save my project cubase crashes.
Canh you share your system details? OS, processor and RAM, etc?
What other plugins are you using alongside Reaktor?
My system:
Win10 x64, Cubase9, AMD A10-5800K, 12G RAM
Other plugins are, HSSE, EZdrummer2, U-he Diva, Xpand!2
I don’t think the other plugins are causing it.
You can easily reproduce this on an empty project with only one instance of reaktor (blocks).
Load up a random blocks ensemble, enable automatic saving (little ‘host’ icon lights up)
make a couple modifications, then little disk icon will light up white meaning modifications will be saved with the DAW project.
Ctrl+S and there you go cubase crashes.