For each participant you should get a .csv file and a .log.gz file. Perhaps the syntax error is associated with the log file, rather than the csv file.
I would not recommend manually editing the file name on Gitlab, but I’m not sure if that’s what you said you were doing. There has been a recent change to the access of CSV files on Gitlab to reduce the affects of AI bots.
You could investigate the error further by using the Developer Console.
See also:
Perhaps you are ending up with multiple participants trying to use the same filename.