As a first step, could you please check if you're running the current version of the mobile app and if you're not, try re-installing it? You can find the relevant steps here.
Please note that uninstalling the mobile app doesn't delete your files in your Dropbox account but any offlined or cached files will be removed from the device. For this reason, check that any changes made to offline files have been synced to your account and, if you'd like to make them available offline again, take a note of them before reinstalling.
If you're still having trouble uploading the video file even after this, could you please ensure that you're connected to a strong and stable WiFi connection, or try connecting to cellular data or even a different WiFi network?
For this issue, maybe you could try to reformat the video files and then try it again. If you want to do that, you could use the tool DRmare Media Converter Mac to help you. Hope it will meet your needs.
Hi. I have this same problem but uploading voice recordings and mp4's from an iPad. I am running the latest version of Dropbox. Please can you send me the advice that you gave to this previous customer so that I can hooefully fix this problem? Thanks