SOLUTION: Use Chrome Incognito or Firefox (both worked for me on Mac) and follow these guidelines (by cryptosergio):
- The Snap image must be uploaded while the video is uploading.
- The video must be below 1Gb.
- You need to wait until the video has been uploaded to submit (it can take quite long depending on the network)
I tried many options and finally got a video uploaded with this method.
edit: Upon trying again it logs me out and prevents me from uploading sometimes still.