@ChrisElls If you mean the airplane mods you installed, no they didn't cause your system to require activation. It is more likely a temporary server or connection issue. Which version of game, Steam or Epic?
However it's also possible, given you've reported crashing your game multiple times that you corrupted your save game file or other game files. If verify doesn't fix the issue, and if it's not a temporary connection issue, delete your game save files.
Finally, if nothing helps, use Google search to find possible fixes.