You shouldn't need to specify the WebURL, and in most cases you wouldn't want to anyway. The userdisp.aspx page shows values for the user in the current Site Collection.
I just tried the function on my Office365 instance (same as you; the back end is 2013, but I see 2010) and it works for me.
M.
I just tried the function on my Office365 instance (same as you; the back end is 2013, but I see 2010) and it works for me.
M.