This behavior can occur when the Outlook.exe process was not properly shutdown when Outlook was exited. Even though it may seem like Outlook has shutdown (as the User Interface is no longer visible), the Outlook.exe process may still be running in the background.
You can verify this via the Task Manager: When Outlook is closed, go to Task Manager >Processes and check if Outlook.exe is running. Wait for it to shutdown, and then start Outlook again. Then the E-mail Convertertoolbar should be visible.
(It is normal for Outlook.exe to run for a while before it shutdown itself. 7-8 seconds is normal. But if it keeps on running, then the only way is to force it to shut down by ending the process in the Task Manager.)