Answer by Prayag Verma for Error parsing XML, line 744, column 97: The entity "u" was referenced, but not declared

The problem is being caused by the & symbol present in the script tag's src link. You will need to escape it by replacing it with & to make the script tag get accepted by Blogger's XML parser -

<script type="text/javascript" src="https://js.adnetwork.com/jp.php?c=b4e403v2x256r2w2s2b43384&amp;u;=http%3A%2F%2Fwww.adnetwork.rocks"></script>