I had the issue in the past but wasn’t able to reproduce it. However, just this morning after updating to 4.0.1 and restarting Knime, my whole OS (Mac OS X, except mouse movements, freezes for +10 seconds.
To resolve the issue in the past I had to completely reinstall Knime and all it’s extensions. Can anyone reproduce it?
I have the most recent version of OSX v10.14.6 (18G87) in use. Starting KNime from terminal with -clean parameter resolved the frozen system issue. Does this help you?