Outlook Desktop will not open if Onenote Desktop is already open

Nothing happens when i click the outlook icon.

If Onenote is not opened on Windows Startup, i can open Outlook and thereafter Onenote, but not the other way around. I cannot open Outlook, if Onenote have been opened.

A Workaround is to shut down all Outlook-processes from the taskbar and then open Outlook again.

What is the bug?