mp4 files won't play after dropbox upload/download

I have created a twenty five minute video in iMovie. Using iMovie, I saved a smaller, 540p, version of the file to upload to dropbox. (While the ful-sized save of the video is in .mov format, iMovie saves 720p and 540p versions as .mp4 files. The saved file opens immediately and plays well in quicktime (or VLC). However, the same file, when downloaded from dropbox, gets the "Quicktime player can't open..." message.

When trying to open the downloaded file in VLC, the program acts strangely: it does not open a video window at all, but in the VLC "main window" the file is listed with a running time of 12:55:57, which has no connection to the actual running time of roughly twenty five minutes.

I have uploaded and downloaded the file repeatedly, so the problem isn't some sort of one-time file corruption.

Please advise! Thanks.