TLDR: Too many video uploads within 5mins.
So I was running into this issue today. I started uploading videos and I had a bunch of videos I needed to upload. I don't know if my issue is exactly related to yours, but mine had to do with how many videos I was uploading in 5min intervals... and their error message needs to be altered to reflect that.
When I started getting the error message I looked at the JavaScript console to see if it was outputting some useful error messages because unknown error is a ridiculous message. All I could see was an HTTP 500 error and CONNECTION_TIMED_OUT. So I looked at the network traffic, yeap... there's the error:
{"error":"server_error","error_description":"( now - auth.last_root_post ) > STEEM_MIN_ROOT_COMMENT_INTERVAL: You may only post once every 5 minutes."}
The backend server(s) are sending proper error messages, and the frontend isn't catching those error messages and displaying it for the user to see. Really simple fix on their part, just don't know why they aren't doing it. Hope that helps!