This error was driving me NUTS for hours while trying to xls transform a complex xml document with the help of inline javascript:
“Exception: Extension function parameters or return values which have Clr type ‘ConcatString’ are not supported.”
Thank God someone else already had the same problem. The solution is rather weird, but the return value is now correctly recognized as string.
See the full solution here:
http://www.rendelmann.info/blog/2006/12/01/Strange+XSL+Transform+Exception+Figured+Out.aspx
As a sidenote, MSXML 6.0 does NOT support XPATH 2.0 at the moment…