bug for .csv download

Hello,

I use Form Manager version 1.7.0
When I try to download my form datas, i have this message :

Warning: Cannot modify header information – headers already sent in /var/www/hubspitzberg2015/wp-content/plugins/wordpress-form-manager/getcsv.php on line 36

Warning: Cannot modify header information – headers already sent in /var/www/hubspitzberg2015/wp-content/plugins/wordpress-form-manager/getcsv.php on line 37

Warning: Cannot modify header information – headers already sent in /var/www/hubspitzberg2015/wp-content/plugins/wordpress-form-manager/getcsv.php on line 38

Warning: Cannot modify header information – headers already sent in /var/www/hubspitzberg2015/wp-content/plugins/wordpress-form-manager/getcsv.php on line 39

What can i do ?

Thanks for helping

https://www.ads-software.com/plugins/wordpress-form-manager/