Error about saving data: [jspsych pavlovia plugin 3.0.6] error

Data of most participants could be saved successfully. However some participants may meet this error at the end of the experiment, and I could not find their experiment data. The screen shows:
when uploading the participant’ results for experiment
when saving data from a previously opened session on the server
unable to JSON decode the value: Extra data: line 1 colum 13 (char12)

Thank you very much!