Then something is missing or corrupt, or your game half updated with the recent patch. Hit Re Update on the launcher and then complete the installation again. I'm puzzled as to how so many people have issues launching the game.

1. Install mod on clean d2 version.
2. Finish required updates and make sure the extraction actually takes place (sometimes an AV can remove files on extraction causing the error)
3. Restart your pc (not necessary 93% of the time, do it for good measure)
4. Try to run the game via the launcher. If good, HURRAY
5. Unhandled Exception? Uh-oh something went wrong:-->
6. Re Update your game, this time make sure you don't have anything blocking false positives and that the extraction process is 100% COMPLETE (to the correct directory - usually "./" works.
7. Launch game, it will work.