Some players have reported that using the direct connect command in the game's console can sometimes bypass the error for specific servers.
False. Even Steam users get it when joining modded servers with custom protection.
In Steam properties, ensuring you are for both Half-Life and CS 1.6 can sometimes resolve versioning bugs. Manual Patching (Non-Steam):
This problem happens when the client.dll file inside your local game directory does not match the exact file version running on the host server.