returning HTML in $event->sender->addErrors doesn't work
Allright, after digging &amp; debugging a bit further into this issue I found it out myself!
$event-&gt;sender-&gt;addErrors([
    'expiryDate' =&gt; Template::raw('&lt;a href='.$entry[0]-&gt;cpEditUrl.'&gt;'.$entry[0]-&gt;title.'&lt;/a&gt;')
]);

PS: don't forget to add this to the top of your php file:
use craft\helpers\Template;