I'm using a custom aspect ratio when I export, 1920x817 or 2.35:1
I click preserve using crop, and export. The QuickTime file looks fine, no line. It's only when I upload to Google Drive or DropBox that a white line appears running across the bottom.
Yeah video looks terrible on Dropbox and you have no control over the compression. You're better to either make your own mp4's (I use Compressor) and upload to an FTP and share the link, or upload to Vimeo and share that.