I have the same issue today. Doesn't matter which video. I tried a few and they all give me "Unknown Error."

I had it in the past (about two weeks ago), too, when I last tried to upload something; but eventually, it would accept the video after I waited for about 5 minutes before hitting "Submit" again. But today no luck with waiting either. Does the error still persist on your side?

Edit: Hard to believe ... but in my case, logging out from d.tube and logging in again resolved the issue.