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.
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.