My first impression was that this might be a Chrome issue. I have encountered some problems which occurred with Chrome on Linux and not with Chrome on Windows, so it is certainly possible that this is specific to Chrome on Mac.
Maybe there are records in the table which were created in a version of WW before 2.15 which were put in the database in some "bad" way, and now cannot be retrieved.
If someone there feels comfortable directly modifying the database, I would recommend trying to "removing" the records for the specific user and specific problem and testing to see if that helps. (I'm recommending changing the user_id field, as then in
the future we could look in more depth at what is actually in the table which is making trouble).
Something like the lines below - fixing the table name, student user_id, set name and problem_id (a number)
select user_id,set_id,problem_id,count(*) from CN_past_answer group by user_id,set_id,problem_id;
select * from CN_past_answer where user_id='the account ID' and set_id='set name' and problem_id='N';
update CN_past_answer set user_id='REMOVED_FROM the account ID' where user_id='the account ID' and set_id='set name' and problem_id='N';
Warning: I typed that SQL code after running similar commands, but could have messed up the syntax when editing to make generic.
If removing the records fixes the problem, we would have a better understanding of the problem