I am using Movie Studio 18, although I ran into the same issue with Movie Studio 14. I personally regret buying Movie Studio 18 after finding out this problem has yet to be resolved. The only reply I get is some arbitrary reply about how most people don't even use this function, which is beside the point and not helpful in the slightest. Not to mention because I used the product key I'm not allowed to return this program (which seems unethical in itself).
When I go to upload a video to youtube, it prompts a small window with the google login and verification, after entering the correct credentials, I am always met with the same error window stating there was some issue from the server-side of the client. It seems either the window that Movie Studio 18 initially opens is extremely outdated and cannot run youtube or the program itself is not up to date with the proper verification to get past YouTubes firewall.
I have made requests about this in the past and have received unmotivated responses in the direction of "Movie Studio not being able to keep up with youtube terms of service" which is completely lethargic. It seems whoever is in charge of that area has failed to do their job. I don't see any other sites having issues being able to upload to youtube. It seems to me this is solely a Movie Studio problem and I suggest you fix it if you want to keep your customers happy. As soon as I noticed this was STILL not resolved I immediately ran for the door. But silly me didn't read the fine print of your "terms of service". Saying I can't return an item that I purchased fully. I don't intend on using this product if I am not able to use the simple function of uploading. I don't know what kind of "legit" business would not allow a return on an item from a customer that has only used it for 2 hours. You can state your legal codes all you want. I know from many other sites being able to return an activation code is no issue. It's unethical at best.