This problem could have different reasons and depends on the used Windows operating system

Windows XP / Vista / 7 / 8.x

Note: This poblem is only related the 64Bit versions of the Windows, because these versions only allow to install kernel drivers which are signed by the publisher of the driver.

...

"Windows cannot verify the digital signature for the drivers required for this device. ..."

Solution

Note: The KB fix must be installed before installing the driver.

• • Users of Windows 7 should check if KB3033929 is installed.
    The fix can be downloaded under the following link: https://technet.microsoft.com/en-us/library/security/3033929.aspx
  • Users of Windows 8.x should execute a "Windows Update".
  • Users of Windows XP 64Bit and Windows Vista 64Bit must update their Windows system to Windows 7 or later,
    because Microsoft does not offer any upgrades to support the new type of ceritificates.
    
    

Windows 10 - 1607 and later

Microsoft has changed the signiture checking inside of WIndows.
If the PC BIOS is set to "SECURE Boot", Microsoft only allows to install driver which are signed by Microsoft.

...

"Windows cannot verify the digital signature for the drivers required for this device. ..."

Solution

• • The ONLY way to solve this problem is to DEACTIVATE Deactivate "SECURE Boot" in the PC BIOS.
    

Note

• • 
    We are working on an update of the driver signiture but therefore a new Code Signing Certificate is necessary and
    the signing proceedure hat to be changed.
    As soon as we have setup this new proceedure we will offer an updated driver and this should be done until 07/2017.or
  
  
  • Download the updated "cifX Driver Media CD - NXDRV-WIN CD V1.3.0.0 W10_SecureBoot.zip" (link: V1.3.0.0 (NXDRVWIN)),
    uninstall the actual driver and install it again from the updated media..

Windows 10 Server (Server 2016 and above)

Windows 10 Server (Server 2016 and above) is NOT supported by the cifX Device Driver for Windows