So you use the "upload without progress bar"? I use that as well, and yeah, I noticed that error too. It's an error of YouTube, and just as always, they're pooping on the errors, but maybe it will get fixed soon.
I hate uploading videos on Youtube. EVERY SINGLE TIME I try to upload a video, I go through at least 10 "unknown errors" while uploading, and after what seems like OVER 9,000 TIMES of trying to upload again and again, it finally uploads.
I'm still trying to upload a video I made yesterday.
YouTube is just completely broken beyond belief! I keep getting the message "an error has occured please try again later." They keep "updating" the site but they still won't FIX THE DAMN THING in the first place!