I can only advice you the usuall:
1. Install the modpack as administrator (in certain cases you have to right click on .exe and select as admin...), the game should be played also on admin account.
2. Make sure you're installing to correct folder (some people have multiple installations).
3. Refrain from using special characters and multiple spaces in the path to game installation.
4. Make sure no antivir is interrupting the process.