+1 for the patch in +9.
It worked perfectly to provide new operator options "Is empty (NULL)" and "Is not empty (NOT NULL)" inside of the Views Filter Criteria section.
I was getting this error with the ResponseText: parsererror. My problem turned out to be business rules I had set up with the Business Rules module. I forgot that I had these rules that redirect to a certain page after a record was created for various content types. I was able to disable those rules while I was importing data and this solved the issue for me.
I wanted to share my experience in case anyone else has been banging their heads trying to figure out why they keep getting this error. Consider that it may not be the Business Rules module in particular, but you might have another module installed that is conflicting with the import process. I hope this helps.
tylired → created an issue. See original summary → .