No midi editing

Both my brother and I have new mac-mini, and the latest Cubase 10.0.4., and have 20 years experience, going back to the Atari.

Trouble when I start to edit a midi track, just deleting an early single note. Putting the eraser on note, and clicking turns the whole track grey and all notes disappear. When I dropbox the file to him, he can erase note.
When he sends me file back I have the same problem.

Opened a song created 2 days ago, same problem.

Does anyone have any suggestion?

Don Winfrey
Manhattan Beach, CA