Somebody using you activation code,or by mistake you using someones.
My friend was mistakenly typed my activation number and entered on the server with 1.2.7.beta.So,I was able to enter on the servers with 1.2.6. version, but with 1.2.7.
You need to go at www.digitalcombatsimulator.com, log in with your username and password.Top left you have links.Click at Personal section-Licensing.
In the blank field, enter your activation code.You can find it at DCS World folder/mods/aircrafts/flaming clifs/bin, fc3_protect.exe file.Double click on the file,chose activation/deactivation-continue.Your activation code will be shown and selected.Copy number and cancel activation/deactivation.Enter it at at empty field on the Licensing page,press Request information.
You will get all information who and from where and how many times has activate the game with that activation code.( on the bottom )
If somebody of your friends using it,he must enter at the same page with his user name and password,and chose to Bound that activation number to your nick name,the nick name under which you are loging in on the www.digitalcombatsimulator.com and master server at multiplayer of the Flaming cliffs.
Hoppe I was clear and this will help.