Do you have a proposed CSS rule that could hide this nag?
This seems to be the element:
<div class="custom-sidebars-box custom-sidebars-checkup">
<div class="cs-inner cs-wpfssl">
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<div class="wpfssl-logo"><a target="_blank" href="https://www.fsmuw.rwth-aachen.de/wp-admin/plugin-install.php?s=webfactory%20force%20ssl&tab=search&type=term"><img src="https://www.fsmuw.rwth-aachen.de/wp-content/plugins/custom-sidebars/assets/img/wp-force-ssl.png" alt="WP Force SSL" title="WP Force SSL"></a></div>
<div class="textcenter"><a target="_blank" href="https://www.fsmuw.rwth-aachen.de/wp-admin/plugin-install.php?s=webfactory%20force%20ssl&tab=search&type=term" class="button">Install the <b>free WP Force SSL plugin</b></a></div>
</div>
<div class="cs-inner">
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<div class="sticky-logo"><a target="_blank" href="https://www.fsmuw.rwth-aachen.de/wp-admin/plugin-install.php?s=webfactory%20sticky&tab=search&type=term"><img src="https://www.fsmuw.rwth-aachen.de/wp-content/plugins/custom-sidebars/assets/img/wp-sticky.png" alt="WP Sticky Anything" title="WP Sticky Anything"></a></div>
<div class="textcenter"><a target="_blank" href="https://www.fsmuw.rwth-aachen.de/wp-admin/plugin-install.php?s=webfactory%20sticky&tab=search&type=term" class="button-primary">Install the free WP Sticky plugin</a></div>
</div>
</div>
Which plugin is this concerning? - Most likely https://de.wordpress.org/plugins/custom-sidebars/, but could also be caused by https://de.wordpress.org/plugins/classic-widgets/ Upload a screenshot from your WordPress admin where the nag is clearly visible From the widgets management page:
When does the nag appear?
Do you have a proposed CSS rule that could hide this nag? This seems to be the element: