What a coincidence! I had the exact same issue yesterday. For the first time in my life, I tried to create a YouTube channel and "share" a video from iMovie to Youtube, and it worked for a while and then gave me the same error.
Googling this took me to a few Apple discussion groups where people had the same issue and there was no resolution.
So, I just gave up and "shared" with Vimeo instead, and that works fine, except that I cannot upload a 1080p video. Even if my settings say 1080p, it only uploads a 720p video.
I am guessing it's because Google and Apple are no longer BFFs?