Problems website after installing petition

I have installed the speakup email petition (https://www.huisdieren.nu/over-pvh/tegen-positieflijst/), and it works perfect. But since the installation several other problems occured. Sitemap is not possible (I tried several plugins), Chrome does not show the website at all, Logging into the website is not possible anymore in IE and Chrome (FireFox still works). All these problems show the same error:

Warning: Cannot modify header information – headers already sent by (output started at /www/htdocs/huisdiec/wp-content/plugins/speakup-email-petitions/includes/emailpetition.php:1) in /www/htdocs/huisdiec/wp-includes/pluggable.php on line …

How can I fix this problem? It would help a lot! Thanks.

https://www.ads-software.com/extend/plugins/speakup-email-petitions/