Ampersand is a special character in HTML that indicates the start of an escape sequence (so you can do something like ©to get a copyright character, etc.). If you want to display an ampersand, you need to avoid it. Therefore, if you replace all ampersands with &, this should take care of the error.
However, if at your input there were ampersands that were already escaped - for example, perhaps your data had ©- you will not want to avoid this ampersand. But if your data will not contain any of these ampersands, a simple replacement should be good.
You also need to replace the <> characters <with a larger and smaller character (> and <).
Telerik / http://www.telerik.com/help/reporting/report-items-html-text-box.html
HTML ( XML ) "&" , "<" " > " ( ), , . , "&" "&" . w3.org.