For some reason the space bar has stopped working for playback controls on the timeline or viewers. I installed 15.4.1 a couple of weeks ago but had so many issues I rolled back to 15.4.0. But now the space bar doesn't control the stop/start function on the timetime or the source/program viewers.
Mac Pro 5,1 Mojave
Trashed the preferences
Checked the keyboard preferences
Tried using a default Adobe workspace
Copied the timeline to a new sequence
Created a new project and impoted the old one
Checked the default keyboard settings in OSX preferences.
Disabled the second monitor to see if that was the issue
Shut Premiere, cold booted the Mac (several times)
Does it work if you create a new project (do not import any media) and add Bars and Tones to it? What are your system specifications (CPU, GPU, RAM, OS)? Also, what happens if you switch the renderer to Software Only?