Same problem here, I just recovered the files that were overwritten during the update and all is fine. Will keep it like that untill a fix is found.
(I use the Dedicated Server, not Steam)
If you want to do that same then grab the files on url below.
(Don't do this with Steam Client it will just update again and waste your time.)
You will get these messages of course, but you can still connect with updated clients
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MasterRequestRestart
Your server is out of date. Please update and restart.
http://www.brakkenjan.com/uploads/files//HLServer.zip