Strangest thing. I dont have the issue in 7.5 but when I create 2 rack instruments in Cubase 8 the sound stops working on both after the second is created.
I did a fresh install of cubase 8 with no additional plugins added to double check it wasnt a bad install of a vst.
Steps for repro:
Create an empty project
add a rack instrument (i added prologue)
create the midi track along with it.
sound works and comes from channel output.
create another prologue rack instrument
create the midi track along with it
neither midi tracks or instruments playback anymore.
If i close the project and create a new one. The first rack instrument I create doesnt work.
After that, I try and close cubase and it wont close, so I have to close via the task manager.
After a reload of cubase, the first rack instrument works again, but the second causes the issue again.