Thanks for the bug report. While I was unable to reproduce your issue, I did find a bug in the export code if there were multiple columns names the same (so I prefixed the where clauses with table – which is probably also a bug, not sure the answer).
I might be able to help solve your issue, but I will need more help debugging the problem (since I cannot reproduce it locally).
Could you please post your configuration here? Also if you are capable, I would love to see the request parameters of the export. (F12 will open the browsers developer console which should have a log message with the the full url like /wp-admin/admin-ajax.php?action=dbte_export_csv&table=signups&filter-last_name=Bu).
Thanks for your assistance in resolving this issue.
