Breaks the widgets page in the backend

[twwn]

Oddly, with the plugin active (in mu-plugins), the widget page (wp-admin/widgets.php) breaks almost completely: Wordpress will try to load the ones already set up, but just fail.

The log contains lots & lots of:

PHP Warning:  Cannot modify header information - headers already sent by (output started at /srv/wp-content/mu-plugins/bye-bye-pingback.php:61) in /srv/wp-includes/rest-api/class-wp-rest-server.php on line 1717
PHP Warning:  Cannot modify header information - headers already sent by (output started at /srv/wp-content/mu-plugins/bye-bye-pingback.php:61) in /srv/wp-includes/rest-api.php on line 715
PHP Warning:  Cannot modify header information - headers already sent by (output started at /srv/wp-content/mu-plugins/bye-bye-pingback.php:61) in /srv/wp-includes/rest-api.php on line 716
PHP Warning:  Cannot modify header information - headers already sent by (output started at /srv/wp-content/mu-plugins/bye-bye-pingback.php:61) in /srv/wp-includes/rest-api.php on line 717
PHP Warning:  Cannot modify header information - headers already sent by (output started at /srv/wp-content/mu-plugins/bye-bye-pingback.php:61) in /srv/wp-includes/rest-api.php on line 718

Disabling / removing the plugin makes the widgets page work again.

using WP 6.0, PHP 8.0

[twwn]

Alright, seems the solution is to simply not put it into mu-plugins.

[twwn]

… or not. Since a provider change, the error is back when opening widgets on their backend's page, despite bye-bye-pingback not being in the ordinary plugin folder.

I wager the actions/filters need an appropriate priority?