I just checked on your site and it is working fine. You had no data fields selected to export. You have to select something, like First Name and Last Name for example... the export needs SOMETHING to put in the export file
I had this same error.
In the third column, if you select EMAIL or PHONE it would give you this error.
I checked the database and those fields were not in the dtregisters_group_member file. I added the two fields and "select all" and/or selecting those two fields now work.