A fix for the unknown error! After failing to upload my last video no less than 14 times, I finally got it to work by trying a different browser. Another user told me that he normally uses Chrome and I should try Firefox. However, I am a Firefox user, but I tried Chrome instead and it worked.
There are several theories about this, including that it might be related to cache, cookies, or plugins. Regardless, it seems like trying a different browser from your main one is the way to go. Hope this helps people and prevents you all from wasting as much time as I did!