If you're having trouble uploading with the most recent Mac DropBox, try installing an older version. It might be more compatible with your video coding.
Here's the link for a previous version: http://www.dropshots.com/binaries/DropBoxMac5.0.dmg
Here are the instructions for downloading & installing:
1. Download DropboxMac5.0.dmg from the link provided.
2. Drag the existing Dropbox into the trash from the Applications folder.
3. Delete the DropBox file in Libraries/Receipts folder on the drive where they will be installing dropbox5.0.
4. Then install as usual.