Interesting - I'm not able to replicate this in my 17.1 environment. I have a contact called "last, first", and searching User by Name for just "last" properly returns that contact in both panes. The only time I see that AHD04610 error is if I search for a name that doesn't exist like "lastname", but in that case nothing is displayed on the left pane, and no results appear in the right pane. By chance, have you made any modifications to either profile_menu.htmpl or list_cnt.htmpl? If you open up the search filter with the Show Filter button, do you see any unusual constraints under the Additional Search Arguments field?