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

[gilles]

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 ?
Thanks