Note: Always support the developers. Paying for the Season Pass or GOTY edition guarantees you safe, fast, and reliable access to every Fallout 4 DLC file without the risk of malware or legal action.
A: This means a .ba2 file is corrupted. Delete that specific DLC folder contents and use Steam's "Verify Integrity of Game Files" to redownload only the damaged archive. Fallout 4 Dlc Files Download
Happy surviving, Vault Dweller.