EDIT: Please note, that as of 18/06/2015 this error no longer appears in New Sheets! Looks like google fixed it! ;)
ORIGINAL ANSWER:
This has proved to be an interesting one...
I cannot explain why - the odd bahaviours in google sheets have baffled me for a while...
I have tested this, and your IMPORTXML() formula returns en error, as described by you on google sheets using the "New Google Sheets", but not on sheets that have been created using the "old sheets"
How to tell if a sheet is "old" or "new":
New sheets have (when opened)
- a green tick on the bottom of the screen
- a green "sheet" icon on the tab / window title bar (favicon)
- a "Add-ons" menu item on the main toolbar between "Tools" and "Help"
Old sheets have (when opened)
- no tick
- a 3-coloured "drive" triangle icon on the tab / window title bar (favicon)
- no "Add-ons" menu item
How do you create an Old-Sheet? - Click g.co/oldsheets (here is the original google help article)
Once you've done that, write your IMPORTXML() function exactly as you described