Fatal Error – php

the plugin worked well … but now I get this error:

Fatal error: Uncaught exception ‘PHPExcel_Exception’ with message ‘Invalid character found in sheet title’ in /var/www/web25355923/html/tm-wochenende.de/wp-content/plugins/gf-entries-in-excel/vendor/phpoffice/phpexcel/Classes/PHPExcel/Worksheet.php:467 Stack trace: #0 /var/www/web25355923/html/tm-wochenende.de/wp-content/plugins/gf-entries-in-excel/vendor/phpoffice/phpexcel/Classes/PHPExcel/Worksheet.php(841): PHPExcel_Worksheet::_checkSheetTitle(‘Wochenendkurs: …’) #1 /var/www/web25355923/html/tm-wochenende.de/wp-content/plugins/gf-entries-in-excel/src/Renderer/PHPExcelRenderer.php(84): PHPExcel_Worksheet->setTitle(‘Wochenendkurs: …’) #2 /var/www/web25355923/html/tm-wochenende.de/wp-content/plugins/gf-entries-in-excel/src/Renderer/PHPExcelRenderer.php(107): GFExcel\Renderer\PHPExcelRenderer->setTitle(‘Wochenendkurs: …’) #3 /var/www/web25355923/html/tm-wochenende.de/wp-content/plugins/gf-entries-in-excel/src/Renderer/PHPExcelRenderer.php(39): GFExcel\Renderer\PHPExcelRenderer->setProperties() #4 /var/www/web25355923 in /var/www/web25355923/html/tm-wochenende.de/wp-content/plugins/gf-entries-in-excel/vendor/phpoffice/phpexcel/Classes/PHPExcel/Worksheet.php on line 467