Hey the Samsung marked solution is in fact not a solution - this is. Hats off to SERichards for figuring out that AEServer as well as DeX both need accessability permission set. As usertociiAhDbl pointed out you must restart the DeX App on the host computer.
In my case, this solution did not work.
In my case, what worked was adding the dex application in the accessibility settings.
To do that, just go to:
System Preferences on the mac > Security & Privacy > Choose on the Left Accessibility, click in the little locker in the bottom left corner.
Check the box of the DEX application. Restart Dex and it should work. Good luck.
personally I resolved the problem uninstalling a keyboard extensions on my mac , karabiner-elements.
After that no more problems.
Currently I'm using :
@Home DEX on a macbook pro 2018, macos Mojave 10.14.6 and a keychron bluetooth keyboard (connected to my mac when at home)
@onthego DEX on my NOTE10 the same BT keyboard and a MS BT mouse