My WordPress Installation appends ‘=’defer to all JS URLs.

Now they are not found anymore becuase they obviously do not exist.

<script src="https://example.org/wp-includes/js/jquery/jquery-migrate.min.js'='defer" id="jquery-migrate-js"></script>
<script src="https://example.org/wp-content/plugins/woocommerce/assets/js/jquery-blockui/jquery.blockUI.min.js'='defer" id="jquery-blockui-js" data-wp-strategy="defer"></script>

What can I do to fix that? It’s happening for all kind of JS files.

I tried a lot of functions.php snippets already, but nothing helped.