In my case that was an issue caused be the new "Read Aloud" functionality, as suggested in this thread on Microsoft's site.

You can toggle this feature by:

  • clicking on "Read Aloud" in "Review" menu.
  • using the Ctrl+Alt+Space.

A control panel shows up or disappears at the top right corner of the edited message. When it disappears, the Ctrl+Left and Ctrl+Right keys work as expected.


It's much simpler than all that!

Ctrl+Alt+Space bar toggles the reading stuff.

It's all too easy to hit that combination by accident!