Launcher refuses to download add-ons because it seems to believe that Steam is not running. However, Steam is running and is not in offline mode.
I've tried re-installing Steam.
I've tried deleting the "C:\SteamLibrary\steamapps\common\Arma 3\Launcher" folder and then used "Verify integrity of game cache" via the Steam client.
I've tried running both the launcher and the Steam client as administrator.
("Have you tried turning it off and on again?") Yes, I have tried the obvious solutions like: Rebooting, Unsubscribe, Subscribe, Disabling all Anti-virus software.
I've tried head-butting my keyboard and that still didn't help. ;)