Clicking on a person’s screen name is supposed to take you to a person’s profile page. It will give you that error message for one of two reasons:
(a) The person has a user name different from their current screen name (YWN has a feature where you can change your posting name from the one you registered with [disclaimer – this is not so kosher anymore]).
(b) The person’s screen/user name has spaces in it. The code for the links does not put dashes in between separate words of a user name when it redirects you – dashes that are necessary for a screen name to be identified properly.
Example: frummy in the tummy. Clicking on his link will take you to http://www.theyeshivaworld.com/coffeeroom/profile/frummy%20in%20the%20tummy (profile/frummy in the tummy), when you really need to go to http://www.theyeshivaworld.com/coffeeroom/profile/frummy-in-the-tummy.
The first cause is pretty hard to get around; the second is simply a matter of entering the dashes into the address bar.