As of October 2013 Steam updated to the new Steampipe format which causes clients attempting to connect to a non-Steampipe server to receive the "Steam Validation" error. Servers must update to the new Steampipe format to restore compatibility with the mod.
Due to this update, server admins will only be able to run dedicated servers on Windows as support for the 2006 SDK has not be released for Linux and may be dropped. Many of the current servers (most of which were Linux-based servers) unfortunately had to be dropped. The remaining servers running under Windows should now be updated to the new Steampipe format.
If you are running or would like to run a Windows server, we are updating the website with the latest information on how to setup a Windows-based dedicated server on the website.