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.