If it's a new project, you should get the latest version. There have been a lot of bug fixes.
But pag is right.
One case where may happen is if the animation is playing. During playback, you can't select/move the mesh vertices.
You may not notice that it's currently playing back if your current animation has only a column or no frames and it's looping.
But you'll know it's currently playing if the Play button is highlighted. Just click on the play button or press D
on your keyboard and it will toggle between play and pause.
The play state also persists when you switch between Animate and Setup Mode.
The editor should probably handle this case properly.
@pag love this.