We don't want to play the blame game, but yes: Valve did it again.
Posted by [ladnet]brad on Oct 8th, 2013
The legacy continues: Valve pushed an update to Source SDK Base 2006 as well as Source SDK Base 2007 which at first breaks each and every Source 2007 based mod. If you try to start AHL2v2 right now, you'll briefly see the main menu background, then you'll be back at your desktop with an "Engine Error" message. Action Half-Life 2 v2 relies on Source SDK Base 2007 which itself relies on Source SDK Base 2006. The latter got converted to SteamPipe, which happened on my system after I forced the paused update to continue two times. Source SDK Base 2007 is seemingly supposed to be converted to SteamPipe as well, there's a SteamPipe beta up for it you can sign up for without the need for a password. Opting into that beta solves the issue for me. If you want to play AHL2 right away, you will have to make sure that Source SDK Base 2006 gets converted over to SteamPipe and opt into the SteamPipe beta for Source SDK Base 2007. Otherwise you will have to wait until Valve pushes another update, which either moves the SteamPipe version of Source SDK Base 2007 out of beta status or fixes the issue another way.
Sorry for the inconvenience, but there's nothing else we can do about it.
Update: apparently the SteamPipe beta left its beta status and is the currently version now. That is, after everything has updated itself on your system, it's back to normal. You can play AHL2 without the need for any workarounds.