1. Launch Arma 3 through the launcher (Vanilla, no mods)
2. Click "APEX PROTOCOL" (Middle selection box)
3. Select any mission
4. Click Play / Host / Join
Crash with error
**RESOLVED**
Re-installed the game (twice) and it now works.
1. Uninstall the game.
2. Make a backup of Arma 3 folder in Documents.
3. Delete both Arma 3 folders in Documents and Steam:
\Users\*YOUR USERNAME*\Documents\Arma 3
\Program Files (x86)\Steam\SteamApps\common\Arma 3
4. Reboot your PC.
5. Run Steam as Admin, download the game.
6. Try to run Arma 3 (without mods).
Thanks to Jiri K at the BIS support team :]