I just installed and activated “A Sub Site Teaser” widget. Afterwards, when trying to make any changes to my site in WordPress, I kept catching PHP errors, like so:

Warning: Cannot modify header information - headers already sent by (output started at /www/htdocs/w0070969/wp-content/plugins/a-sub-site-teaser-widget/a-sub-site-teaser-widget.php:1) in /www/htdocs/w0070969/wp-admin/post.php on line 235

Warning: Cannot modify header information - headers already sent by (output started at /www/htdocs/w0070969/wp-content/plugins/a-sub-site-teaser-widget/a-sub-site-teaser-widget.php:1) in /www/htdocs/w0070969/wp-includes/pluggable.php on line 1121

Deactivating the plugin makes the errors go away, but they reappear on reactivation.

https://www.ads-software.com/plugins/a-sub-site-teaser-widget/