Hi,
did the fresh install just help?
I have the same problem.
I installed DCS World and I was able to go to Multiplayer.
Then the A10C module for DCS World.
I activated using the key.
In the module manager, next to the A10C module I have this info: No Key.
When I try to run Multiplayer from DCS World, I'm getting "invalid serial key".
Thanks!