To reproduce:

• create a new customfield
  • add a value that includes an ampersand.
    Example: C&C
• create a new issue ticket
  • under the newly create custom field, choose the value with the ampersand
• within Issue Navigator, make a search to include the newly created issue
• at the top right, click: Views > XML
• Error, as shown by screenshot below

Note: the screenshot of the issue navigator search shows the C&C value in the customfield 'Single'. It is not necessary to show this column, the error will still occur if column is not there.