In the case the network issues prevent the regular uploads from working, we also save a copy of each participant's tracks to their computer's local hard drive. In order for this to work, each track must have an open connection to the local storage api.
The application was unable to open a connection to the local storage database. This can happen if the user has no disk space available or if they are running their browser in Private Browsing or Incognito mode.
If the participant is running their browser in Private Browsing / Incognito mode, have them fully exit the browser and open it back up in a non-private mode. Also, double check that you are able to see that they have 2GB+ of storage space available in the footer under their track.