Assuming your problems are with the AVCHD (approx. 8 GB) release of RotJ, then you should find a checksum utility inside the directory. Hopefully this small program hasn’t been corrupted. Let me know if it has, and I’ll PM you a link to it. Have you tested the checksums of the ISO and MDS files against those provided in section 6 of the Ultimate Guide?
If the checksums match, all I can suggest is that you try a different extraction program. I use 7zip and have extracted all of Harmy’s stuff without problems.
If they don’t match, then go to the individual rar files you downloaded, obtain a SHA-1 checksum for each one, and PM me the details. I’ll compare them with mine and let you know which rar file(s) is corrupt. With a little luck you might have more success downloading the problematic file second time around. Otherwise, I shall stick it on Mega for you.