Hi JC,
I'll try to help you the best I can
I would re-install CVAcars in a different folder, like I said in my previous post mine is in my FSX folder and see if that solve your issue.
If it does, time for a beer
if it doesn't...we'll have a beer anyway and keep trying 
Next try this:
-Open the CVAcars folder and right click kACARS.exe
-Select "Properties" and then select "Compatibility" TAB
-Select "Run this program in compatibility mode for" Win XP (since it was working for you.
-Select "Run this program as an administrator"
-Apply
Give that a try and see how it goes.
To get full permission:
-Right click on CVAcars folder
-Select "Properties" and then select "Security" TAB
-Select "Advanced"
-Select "Change Permission"
-Click on the user you want to edit and select "Edit"
-Click on the first box (FULL CONTROL) "Allow" and then OK
That should do it, you should have full control/permission on CVAcars (I hope)
Let me know how it goes JC.
Pat
