I had this same issue. After some investigation I found out leading and trailing spaces in some answers in the question bank were causing the error. You can see if you have any problem answers by using a MySQL query. Here is the one I used to find the problem database entries:
SELECT answer FROM `mdl_question_answers` WHERE answer LIKE " %" OR "% ";
Note - the syntax is "<space>%" OR "%<space>"
If there are any answers in the mdl_question_answers table that have a leading or trailing space, you can clean them up with the following UPDATE command using TRIM:
UPDATE mdl_question_answers SET answer = TRIM(answer) WHERE answer LIKE " %" OR "% ";
After cleaning them up I was able to import between classes without errors.
Hope this alleviates your problem.
Sam Alexander