Do you want to upload a sample cpr so I can confirm if it’s something to do with the project??
I’d guess it will probably work here which probably leads on to typical suggestions of resetting prefs or reinstalling Cubase…but might be worth confirming.
NB: I think you’ll need to zip the cpr to allow upload…only needs to be a single midi event of course.
Of course you can also try in safe start mode to see if the prefs corruption is the problem.