“Failed to Fetch” errors usually occur because something disrupted the experiment when it tried to ping data back to the pavlovia server. In my experience this is most commonly caused by a drop in internet connection.
As of PsychoPy 2024.1 you could turn on periodic data saving on your pavlovia dashboard, which means that data will attempt to ping back to the server periodically throughout the experiment rather than at the end of the experiment. This might be something to try to prevent data loss, but in general a simple approach is also to encourage participants to check that they have good stable internet connection.