Source: The error originates from the XML class, but you may see it in other classes such as REST, SOAP (which use XML internally).

Explanation: XML expects that all opening tags have a corresponding closing tag. In other words there must be a closing </p> for every <p>. Whatever input data was passed to XML did not contain a closing tag for one or more elements.

Troubleshoot: If you're using XML, then check that your input data is valid XML. When using a class that sends a network request, check the network response. Make sure it is valid XML.