Metin2 will not start: the patcher and launch errors, the graphics device message, and the mark folder fix
Metin2's problems split into two groups: the patcher will not finish, or the game starts and then refuses to run. The official wiki keeps a troubleshooting page for both, and it is the closest thing to a canonical answer list. It has not been edited since 2015, so read the OS-specific steps as legacy and the generic ones as still worth trying.
The patcher will not finish
Two errors show up here: Could not unzip file. Error occured while downloading, and url/metin2torrent.config.xml error code 2.
For the unzip error the wiki says to make sure no other Metin process is running, restart the computer to be sure, put every Metin folder on the whitelist for both antivirus and firewall, and disable data execution prevention on Windows 7 or Vista.
The config.xml error code 2 is usually a connection problem. The wiki lists a missing internet connection first, then something blocking Metin from reaching the internet, then the same firewall and antivirus whitelist, then disabling offline mode in Internet Explorer.
The game will not start
Three messages are documented.
Metin2.CREATE_NO_APPROPRIATE_DEVICE: close all Metin2 processes, restart the computer and run the game again, then update the video card driver. The wiki adds NVIDIA and AMD control panel steps about removing hooks at Show only TV resolutions.
Sorry, Your System does not support 3D graphics, please check your hardware and system configuration then try again: close the processes, restart, run again, update the video card driver, then update DirectX.
Cannot find python27.dll: install Microsoft .NET Framework 3.5 Service Pack 1.
A separate one: guild logos stop showing
This one is not a startup failure, it is a rendering cache problem. Close all Metin2 processes, go to the Metin2_UK folder, open the mark folder and delete its contents, then run the game. The wiki says to repeat it several times if the first attempt does not take.
What players report
The antivirus and firewall whitelist advice is the one that keeps coming back. On the official Metin2 board, a player who hit Could not locate the game file. Please repair your game was told to use the repair button next to Play and to set the install path in the client settings, and the thread was solved by allowing the Metin2 client through the antivirus real-time scanning instead.
Sources
Metin2 Wiki, Troubleshooting (official Gameforge wiki)
Metin2 UK board, Gameforge client repair error (player reports)