So this issue is happening for me but none of the things mentioned in the forum resolved it. I'm currently using the Genesis theme in wordpress and our export tool does work. We are not on the latest version of wordpress but we last updated in December/January. (that's going to be our next step tonight) The interesting thing about this issue is that we have another almost identical install of wordpress running the same wp/plugin version that has no problem exporting the csv file. Any suggestions would be appreciated, thanks!
http://www.gravityhelp.com/forums/topic/error-when-exporting-form-data-to-csv