Soap UI Pro XML DataSource: error Invalid character entity reference: ;

Converting an XML response from an API stopped working in my SOAP UI Pro XML DataSource Step.

The error is:

error Invalid character entity reference: 

The XML response is a list of patients. If I alter the API call so only the first few patients are returned then I don't get the error when converting the XML to properties.

I write this to return the first patient and it works.

//PatientInfo[1]/Patient[1]

If I call the full list of patients which is 18,000 patients and try to access the first patient using the same XPATH I get the error even if I try to only access the first patient using the same xpath:

//PatientInfo[1]/Patient[1]

Can anyone suggest a way to get past this error so I can test the valid patients?