Hi Jonatan,
It looks like each comment in the Better Recent Comments widget is being wrapped with an additional <a> link, which isn’t something plugin adds at this point. For example, the first comment looks like this:
<div class="comment-wrap">
<a href="https://.../#comment-298075"> <!-- this link is the problem! -->
<span class="comment-author-link">Marek_</span>: “
<span class="comment-excerpt" style="word-break: break-word;">To je v pořádku, každý má jiné preference. Ale myslím, že i aerodynamicky čisté auto může být hezké, např EQS,…</span>”
<span class="comment-date"><a href="https://...#comment-298075">Zář 1, 14:52</a></span>
</a>
</div>
So because you have a link within a link – the ‘comment-date’ link inside the outer one, that causes the error you’re seeing, as it’s not valid HTML.
Could you try disabling the other plugins on your site, or checking for any code which is interacting with Better Recent Comments?