Sorry for missing this post. It seems to be opening the Spelling and Grammar Checker. The default shortcut for that window is ⌘ + SHIFT + :. It is possible that, this shortcut has been manually redefined to ⌘ + /.
Please go to System Preferences → Keyboard → Shortcuts and check whether ⌘ + / is reserved or not.
Thank you for your answer. I tried to restore the deault shortcut keys and it dit no change anything.
Also, I did not found the " ⌘ + / " in my parameters, just the " ⌘ + SHIFT + : " as you can see below.