Please go back to the Edit area of your project file that generated the YouTube upload.
Expand the Timeline with the _+ slider above the Timeline.
Hit the End key of the computer keyboard. That normally would take you to the end of the last item on the Timeline. But, in your case, the Timeline Indicator will probably stop at about 15 minutes beyond the end. You probably have debris, fragments of clips or clips that you moved to the right and forgot about them.
The debris needs to be deleted.
Please verify how you did the upload to YouTube....
a. using built in YouTube feature
b. Timeline export to file saved to the computer hard drive and then the file uploaded to YouTube at the YouTube web site?
The debris may present as thin vertical black lines scattered about the track content. Open all the tracks, used or not and look.
We will be watching for your progress.
Thank You ATR
Thanks for the reply.
Do not give up. Your issue has all the symptoms of leftovers beyond the end of the Timeline content. Sometimes is very hard to detect but you can do it.