I had this problem with the last pieces of two of Harmy's. He suggested to turn JDownloader off for a few hours altogether and try again later, which worked for me. If you just leave it while it says "temp. uncheckable" it makes you wait 10 minutes over and over. Clear the lists and exit JDownloader for a while and try again until the last part shows as online. Then begin the download. Also try opening the dlc with JDownloader through Windows instead of opening JDownloader and then launching the dlc, it probably makes no difference but that's what I did differently after trying for 2 days and it worked.

Thanks for your advice Harmy and fantastic work! Looking forward to Star Wars Despecialized :) Could I please get a PM for the SD version of this as well?