This happened to me, and for a while I had to use a different browser to upload videos to diaries. it has since resolved its self.
Its like a background thing that GD team is trying to do. They are not that skilled in web development given the state of this webpage. But they normally fix most things in a weeks time, and then inadvertently cause a few others.
Other steps to try and clearing your browsers cache, disabling any script/ad blockers and setting privacy stuff to basic or minimum. also trying a different browser or file format. GD convert almost everything uploaded to them so file formats should not be a big issue. I've successfully uploaded JPEG, JPG, GIF, PNG, MOV, MP4, HEIC HEVC.
Feel free to ignore MrSour420, hes a troll that likes to think everyone is scamming shit. Sadly hes a bit stupid and does not realize questions not linked to diaries do not count towards contest totals. However their is a contest that runs once a month for answered questions, its called GOTM and can be found on the contest/giveaway page. It does attarct a lot of misfits and spazes, but I don't think anyone answering your question is that expect him.
growdiaries.com/giveaways