Hello,   
I have a similar problem. I have two accounts on the same email and it generates a problem with an invalid serial key. Please merge my accounts, so that the serial key problem was solved.   
Thanks.   
//Edit:   
Problem solved   
Step by step: 
- Deactivated my A-10 serial key using a10_protect.exe (sample path to the file "Program Files \ Eagle Dynamics \ DCS World \ Mods \ Aircrafts \ A-10C \ bin \ x86_64") 
- Removed network.vault file (sample file path: "WindowsUsers \ YourWindowsUserName \ Saved Games \ DCS \ Config") 
- I logged into the manager license keys and clicked "unbind" (http://www.digitalcombatsimulator.com/myserials.php) - Note: you must log in to your account where you have assigned to the game, which generates the problem. 
- Logout from my second account and logged into my first account, for which I have assigned all the other keys. 
- Launch "DCS World multiplayer" game and re-activate the key 
- After activation, logged into DCS World multiplayer lobby using the data from my first account.   
///SORRY FOR MY BAD ENGLISH LANGUAGE///