Zero-byte file exported from Lightroom

After you export files from Lightroom to a folder, some of the files have a file size of 0 (zero) KB. You cannot open them.

Solutions

Solution 1: Pause Dropbox sync.

See: How do I pause and resume syncing from Dropbox?

Solution 2: Export your files to a different folder.

Select a folder that is not currently set up to sync automatically with Dropbox. 

Additional information

When exporting, Lightroom's first step is to create a file place holder. If you export to a Dropbox synced folder, this place holder is immediately synced to your cloud storage. When this other process takes over the file, it becomes locked. Lightroom can't add new data to it, so it stays a zero-byte document.