Error message

Upon switching to the customizer an error message flashes by

PHP Warning: Cannot modify header information – headers already sent by (output started at /var/www/domains/gerkoper.nl/subdomains/blog/wp-content/plugins/simple-download-monitor/main.php:265) in /var/www/domains/gerkoper.nl/subdomains/blog/wp-admin/customize.php on line 126

Albeit just a warning, it is unsettling to non-experienced users.

Can something be done about that?

Thanks in advance,

Ger