I figured out the problem through simple trial and error.
Using a 500kb clip of Ichigo Mashimaro, I attempted to upload under the very simple title/description/tag "Nope."
I then tried with my IOSYS movie title/description/tag, but with the same video.
I then tried just having the title with SJIS, then the description, then the tag.
All three failed.
Only when I completely cleaned all fields from any trace of Japanese text did the upload finally work.
This would also explain why googling the error only gave Japanese webpages.