Does anyone else have the problem where a effect plugin editor is open and cubase won’t playback? I am asking because maybe there is a setting, or preference that will fix this.
Thanks for any ideas. 
Trash your preferences?
I have had 3rd party plugins that swallow key commands when in focus. On one of them I remember that setting it to NOT being always on top helped.
Or as ggc suggested it could be a corrupt preferences issue.