Error: Cannot unset offsets in CRM_Export_BAO_ExportProcessor->setAdditionalFieldsForPostalExport() (line 169 of /home/username/www/www/sites/all/modules/civicrm/CRM/Export/BAO/ExportProcessor.php”

This appeared to be the same issue that was reported here:

Error “Cannot unset string offsets” exporting using custom field mapping

and so we upgraded their site and that indeed fixed the problem.

We are always here to help.