German language KDE. KMail2 started via Kontact. Selecting "Help" -->> "KMail Introduction" results in an "Introduction" screen which refuses to disappear when any KMail2 Folder is selected. Selecting "Help" -->> "Kontact Introduction" results in an "Introduction" screen which can be closed but, the previously displayed "KMail Introduction" returns and, can neither be closed nor made to disappear. Selecting another Kontact module such as KOrganizer or KAddressbook or Kontact overview, doesn't help. Kontact has to be closed and then re-opened
I use the KDE "Oxygen" Icons and Application's Style Theme. It may be that, there is a button (hidden to to the Oxygen Theme) to close the KMail Introduction -- same as for the Kontact Introduction but, I haven't found it, yet.
Fixed in 5.6.3 or 5.7.0