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Data saving error for small minority of participants

URL of experiment: https://pavlovia.org/ps2/acquiescence2

Description of the problem: I was running a large online study yesterday, and while data collection went smoothly for the most part, there were a few errors that affected a small minority of participants. One of these involved the following message at the end of a session:

Unfortunately we encountered the following error:

when uploading participant’s results for experiment: ps2/acquiescence2
when saving data from a previously opened session on the server
unable to JSON decode the value: No JSON object could be decoded
Try to run the experiment again. If the error persists, contact the experiment designer.

I am confused as to why such a problem would affect a small number of participants, but for most the process went smoothly. Any insight anyone could offer on this—either so I can prevent the issue from occurring again, or just so I can understand why it happened—would be appreciated.