The error message references the last_answer column, so my suspicion is that because of something in the text box (probably an exotic character) WeBWorK is having trouble either writing the past answer to the database, or retrieving it from the database.
I suspect that there needs to be something in the code to sanitize the input from the text box before storing it in the database (or if there already is, it needs to be tweaked). Unfortunately this will probably require someone looking carefully at the code.