Alright, so I had this problem since a few days ago and for me it got fixed by reinstalling steam. Underneath this part I will post several things that might work. Those methods are not thought of by me but by other Steam users.
Here is my method:
1. Install source multiplayer dedicated server 2007 through steam.
2. Download HLDS updater tool
3. Install HLDS to your /steamapps/$USERNAME$/source dedicated server 2007/ folder
4. Open notepad and save the following to "Server_download.bat" (open notepad and save it as that filename, to the source multiplayer server folder)
(REPLACE C:\srcds INSTANCES WITH YOUR FILE DIRECTORY!!! "USE QUOTATIONS!"
@echo off
cd C:\srcds
echo This will take a long time, I would recommend you go do something else.
echo Updating/installing Garrysmod
hldsupdatetool -command update -game garrysmod -dir C:\srcds -verify_all -retry
echo Updating/installing CS:S
hldsupdatetool -command update -game "Counter-Strike Source" -dir C:\srcds -verify_all
echo Updating/installing dods
hldsupdatetool -command update -game "dods" -dir C:\srcds -verify_all
echo Updating/installing hl2mp
hldsupdatetool -command update -game "hl2mp" -dir C:\srcds -verify_all
echo Updating/installing tf
hldsupdatetool -command update -game "tf" -dir C:\srcds -verify_all
echo Updating/installing sin
hldsupdatetool -command update -game "left4dead" -dir C:\srcds -verify_all
echo Updating/installing left4dead2
hldsupdatetool -command update -game "left4dead2" -dir C:\srcds -verify_all
echo Updating/installing episode1
hldsupdatetool -command update -game "episode1" -dir C:\srcds -verify_all
echo Updating/installing orangebox
hldsupdatetool -command update -game "orangebox" -dir C:\srcds -verify_all
echo Update/installation Complete
echo Installed to c:\srcds\
echo Your server is now ready to use!
echo To add extra addons or maps etc. place them in c:\srcds\orangebox\garrysmod\
echo This installer can also update necessary server files, so it is recommended that you run it often.
pause
5. Run server_download.bat, it will update all of the games listed (remove any that you dont want to include). this takes a LONG time!!!
6. there should be a "srcds.exe" in your source 2007 dedicated server folder. Make a shortcut to it. Add "-game garrysmod" under "Target:" so it looks like "dedicated server\srcds.exe" -game garrysmod
7. Run that shortcut. A server startup window should appear, start your server! Validation problem gone!!!!
Here are methods found by other users:
-The list of methods underneath start with easy methods which might fix it, the lowest ones are the ones that might take a little longer because of reinstalling/updating of Steam.
1.Verify the Integrity of the Game Cache
- Open Steam and go to your Library - Games.
- Right Mouse click on the game that gives the error, select Properties.
- Go to the Local Files tab and click on Verify integrity of game cache.
- Try to run your game and join a server.
2.Steam_appid.txt
- Install Source Dedicated Server, it's only a few MB and won't take that much space.
- Start it once, then close it.
- Go to your Steam directory, (C:/Program Files/Steam), and go into the Source Dedicated Server folder. (C:/Program Files/Steam/Steamapps/%USERNAME%/Source Dedicated Server).
- Make a new .Txt file and call it steam_appid.txt
- Open the file and put the ID of the game that gives the error. The ID's can be found by going to the Steam Store and search for the game that gives the error. The number at the end of the link is the ID of the game, for example Garry's Mod: http://store.steampowered.com/app/4000/, fill in 4000 in the text file.
- Save the file
- Try to start the game that gave the error and join a server.
- (If this doesn't work try to create a new folder in the Source Dedicated Server folder called the name of the game that gives the error and insert the text file.)
3.Deleting ClientRegistry.blob
Another fix that might help is deleting the ClientRegistry.blob file in your Steam folder.
- Close Steam.
- Go to your Steam folder, for example: C:/Program Files/Steam.
- Search for the file called ClientRegistry.blob and delete it.
- Start Steam and let it update.
- Try to start the game that gave the error and join a server.
4.Deleting almost all files in your Steam folder
This is an alternative on the fix above which might solve it if nothing else helps.
- Close Steam.
- Go to your Steam folder, for example: C:/Program Files/Steam.
- Press CTRL+A, once all files and folders are selected hold CTRL and deselect SteamApps and Steam.exe, then delete them by either pressing Right Mouse button -> Delete or just by hitting your Delete key on your keyboard.
- Open Steam and let it update.
- Try to start the game that gave the error and join a server.
5.Nothing worked... HELP!
If nothing helps the only thing left to do is reinstall Steam, don't forget to make a backup from homemade maps or addons! This will save you a lot of pain and work.
- Close Steam.
- Open your Control Panel (Start/Windows Sign -> Control Panel).
- In the upper right corner there will be some text saying View by:, click on the text next to it and change it to Category if it isn't set to this yet.
- Underneath the title: Programs choose for the option Uninstall a program.
- Wait a few seconds/minutes until a list of programs shows up, search for Steam, select it, right mouse button and choose for the option Uninstall, follow the windows pop-up to uninstall it.
- Go to the About Steam page(click About Steam) and click on the Install Steam Now button.
- Follow the steps and install Steam. Now install your game(s) and if you have backups of maps or addons you can place them back after you played the game once.
- Start the game, join a server and it should be fixed.
6.It's still not fixed, what now?
If you still get the errors ask yourself the following things:
- Did my Steam Account get disabled by a Steam Administrator?
- Did I buy the game(Steam) or did I download it illegally(Non-Steam)[Please note that you can also buy games in stores, with illegally I mean that you did not pay any money for it IF it wasn't a free game like Alien Swarm].
If you are sure that the two things noted above are not your problems, try to open a ticket at the Steam Support page.
