Do I need ECC RAM for a file server running Windows Server backups?

No. ECC RAM protects against single-bit memory errors, which matter for rendering, scientific computing, or financial databases, but the risk of a bit flip corrupting a file on a file server running nightly backups is laughably small. Non-ECC RAM is rock-solid here—Windows Server 2022 with NTFS has integrity checks, and your backup software will catch any corrupted write on the next job. Save the money for a second backup drive or battery backup instead.