I have a user experiencing this error when exporting to Excel .xlsx format. The error is in the .xlsx file itself:
Fatal error: Uncaught Error: Call to undefined function mb_strtolower() in /var/www/html/lib/external/phpExcel/PHPExcel/Style/NumberFormat.php:448 Stack trace: #0 [internal function]: PHPExcel_Style_NumberFormat::setLowercaseCallback(Array) #1 /var/www/html/lib/external/phpExcel/PHPExcel/Style/NumberFormat.php(464): preg_replace_callback('/(?:^|")([^"]*)...', Array, 'yyyy-mm-dd HH:m...') #2 /var/www/html/lib/external/phpExcel/PHPExcel/Style/NumberFormat.php(647): PHPExcel_Style_NumberFormat::formatAsDate(44811, 'yyyy-mm-dd HH:m...') #3 /var/www/html/lib/external/phpExcel/PHPExcel/Worksheet.php(738): PHPExcel_Style_NumberFormat::toFormattedString(44811, 'yyyy-mm-dd HH:m...') #4 /var/www/html/lib/external/phpExcel/PHPExcel/Writer/Excel2007/Worksheet.php(378): PHPExcel_Worksheet->calculateColumnWidths() #5 /var/www/html/lib/external/phpExcel/PHPExcel/Writer/Excel2007/Worksheet.php(80): PHPExcel_Writer_Excel2007_Worksheet->writeCols(Object(PHPExcel_Shared_XMLWriter), Object(PHPExcel_Worksheet)) #6 /var/www/html/lib/external/p in /var/www/html/lib/external/phpExcel/PHPExcel/Style/NumberFormat.php on line 448
The report does include multiple datetime fields.
