Message viewer doesn't open when I start Mail app on MacOs Big Sur 11.1

The message viewer window doesn't open when I start the Mail app from the dock. The ribbon at the top of the screen shows Mail and its associated drop-down menus. And when I go to Mail>Window>Message Viewer, no message viewer window will open.

When I then use another app and want to get back to Mail by using the dock, the mail app still shows it is open but it doesn't react anymore and does not show the mail app in the menu bar on top.