Safari Upload Bug
There is a nasty bug in the latest version of Safari for macOS (version 11) that prevents uploading to all Flash-based sites. We filed a bug report with the Safari team at Apple months ago, but a fix has not yet been released.
For now the only solution is to use a different browser such as Chrome or Firefox. You can also use our Mac App or Lightroom Plugin to upload.
Update April 2018: Apple have finally release a fix for this bug. If you upgrade to Safari 11.1, uploads will work again.