Zencastr streams each participant's tracks to our cloud servers as they are recorded. In order to do this, each track must have a cloud upload url assigned to it in advance. If these urls are missing, then the uploads cannot succeed.
The participant's tracks were not properly assigned cloud upload urls. Most likely this is an internet connection issue.
Try having the participant refresh the page. If refreshing doesn't help and the recording seems stuck in a failed state, then create a new episode and having them join you there. Also, please report this to firstname.lastname@example.org so we can further investigate the source of the problem.