Iceman, this fixes the issue for me (though obviously running without UAC wouldn't be a solution). The launcher shows "The game is up-to-date" and hitting play launches the game.

I'm not seeing any addons showing in the addons tab - would this be because I have them in a non default location (E:\MYNAME\My Documents\Arma 3)?

Hi, I'm also having this issue. But I'm not getting any of the crashdumps you mention in the Arma 3 root folder. I do have a SteamServiceHost.rpt in the launcher folder if that's useful.