What does this error mean?

This error means an IOException occurred when writing JSON output.

Why does this error occur?

Although the log line says "Failure parsing cell.", the error only happens when writing to JSON output(json.write*) somehow failed (IOException), because the parsing of the cell  was done before calling serializeCell.

How do you fix this error?

The stack trace should provide additional details to the specific cause of the IOException. Use that information to further investigate the issue. If you need assistance deciphering the stack trace, DataStax Support can help.