iMac OS 10.13.6 24G Ram Nuendo 11 Scaler 2.3 (Although it happens with 2.2 as well)
If I have 2 instances of Scaler and one is open, everything seems fine. If I open the second instance of scaler while the original one is open, the Scaler GUI goes black and Nuendo locks up and with the spinning beach ball. Force quit is only option.
ADDENDUM: Turned off OpenGL in Settings and it works fine now. This only happened in Nuendo 11 and Cubase 11.
Thanks for reporting,
Good to know there’s a workaround for now, we will have a look.
Must be the way Cubase and Nuendo handle things that affects Scaler since no other DAW behaves that way on my system. I only found this because I am trialing both Cubase and Nuendo with the idea of side grading to Cubase 11 for film. I do like the workflow in Cubase so maybe although it is picky about some of my favorite plugins.
There’s definitely something strange, I have seen the black screen on the second instance of Scaler in Cubase but it was on Windows.
It never hangs though, I can click on it or move the window and it comes back to life.
Can you please test the Arp mode
Cheers 
ARP works fine here.
Looking around the web it looks like it might be related to NIVIDIA graphics and OpenGL in Steinberg products like Cubase and Nuendo. So my issue might not be Scaler per se.
May i know where to find the open GL settings please.
Hey @muzikboxx and welcome.
You can find the OpenGL settings in ScalerSettings,
You can open it by searching in your Windows Menu or by the finder on MacOS typing ScalerSettings.
Cheers
Georgio