I believe this problem is present only on projects originally create in earlier versions (pre 5.5.3) of Cubase.
Problem: Double clicking the left mouse button does not work ie. to name a track; open midi or wave file in it’s editor.
Cubase is the only program with a mouse problem. I tried three different mice and problem persists.
Work around: Mouse software allows “program specific” button assignments.
For Cubase I assigned one of the two “side buttons” to “double click” when pushed.
It works.
Strange issue though.