XML parsing, illegal character in the end of the string - tsql

Question

I'm getting very strange error when I trying to convert a string to XML in MS SQL Server:
Msg 9420, Level 16, State 1, Line 5
XML parsing: line 1, character 8071, illegal xml character
If I check the string in some text editor, I can see that its length is 8070. Why is it complaining about character 8071 if it does not exist?
This is how I'm converting string to XML:
CAST(REPLACE(SUBSTRING(
REPLACE(REPLACE(REPLACE(ResponseData,'ä','a'),'ö','o'),'å','a'),
PATINDEX('%<?xml%',ResponseData), PATINDEX('%sonType>', ResponseData)+6),
'<?xml version="1.0" encoding="utf-16"?>',
'<?xml version="1.0" encoding="utf-8"?>')as XML) as ResponseData
Are any of replaces causing the problem?
UPD: The problem also is that in ResponseData column the XML string is stored together with some other data. Example:
Error from service: <Some error description>. Sent request: <?xml version="1.0" encoding="utf-16"?><Contents of the XML>
So I need to get that XML string from the column and then convert it to XML.

Answer 1

You could try to change original encoding from UTF-16 to ISO-8859-1, or a more precise encoding for your characters:
DECLARE #data varchar(max) = '<?xml version="1.0" encoding="utf-16"?><...>'
SELECT CAST(REPLACE(#data,
'<?xml version="1.0" encoding="utf-16"?>',
'<?xml version="1.0" encoding="iso-8859-1"?>') AS XML) ResponseData