Magento – Warning: simplexml_load_string() – parser error

If you see this warning, when opening category of your Magento-Shop …

… Of course you could check the XML-Updates in the file:
app/code/core/Mage/Core/Model/Layout/Update.php

The solution

…might be simple.

The reason can be special characters. For example: The &

For that, just exchange the “&”-Sign in your category (title) by the correct HTML-sign “&”