Try changing the Memory Allocator you are using (default: "tbb4malloc_bi" - try "system" on Windows 7 and later).
It may be caused by many issues, such as a hardware malfunction, a virus in the computer, but also an error in the game itself. If the value is 1 then set it to or remove it entirely. Possible solution: In the Windows registry, locate the following key: HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Optionsarma3.exe and check the value of attribute UseLargePages. This type of hack was previously used by Fred41’s custom allocator, but it may cause issues on some computers. This error may be caused by a registry override (a hack) that forces Arma 3 to utilize Large-Page memory. Run the game or the Launcher from a shortcut or from an executable as an administrator.Exit Steam and run as a regular user (instead of running it as an administrator).Run the game from Steam client by clicking on the Play button (not from a shortcut nor directly from an executable).If this situation occurs, Launcher may display a "Updates unavailable - Steam is not running" message in its status. One of the possible reasons for this message is running Steam as an administrator and starting Launcher as a regular user. The libraries with a red icon are the corrupted ones. Download a 32-bit version of the Microsoft Dependency Walker and open arma3.exe with it.Download a 64-bit version of the Microsoft Dependency Walker and open arma3_圆4.exe with it.To identify the precise DLL that is causing an issue.Run the Windows System File Check tool to repair corrupted system files.
However, it still may provide a clue about what the issue is and how to solve it. The exit code in the message is a number provided by the OS and it may be ambiguous. That may mean that the Operating System (OS) was unable to start the game or that the game has crashed. An “Unusual process exit” message is displayed by Arma 3 Launcher whenever it detects that the game was not closed correctly.