No «User login» Dialog Window after pressing «log in» button

  • Hi,
    The version 4.3.0 is installed on Windows 10 64bits, with a french translation.
    (It's an update from the version 4.1.2, and most clients are still in this version)
    When I click on the «Log in» button nothing happens : the user login window does not appear ...
    All other functions I need work fine («Power on», «Restart», «Log off Users», «Run program» and «Open Website»)
    Can you help me to solve this ?