I have added a Grid question (two columns, several lines) and it seems that if I set this question as mandatory, I am not able to continue and I am not able to go to previous. It highlights it in  read and insists the question is not filled out - even though I have one answer in the first column and another answer in the second column. I observe a similar behavior in with the multi-rating questions. Is this the bug or am I doing something wrong? Unchecking the mandatory status makes the survey work...