This is a bug report and a link to the fix.
There are two parts to this;
Part 1 is the toggling of the feature from OFF, to ON.
By default, the feature is set so that Guests (unregistered users) cannot view all posts by an individual Registered User.
Part 2 is fixing a bug that was introduced in UBBT 7.3 (released in May 2008) and and still exists today, in UBBT 7.5.8.
As of UBBT 7.3, if you're a Guest (unregistered user) and you clicked on "View Posts" next to a Registered User's name within the PostList, their MemberProfile, the WhosOnline list, or anywhere else within UBBT, the Guest would be presented with a, "FATAL_NOT_LOGGED" error ("You must be logged in to access this page!") - which is somewhat correct, but...
If you toggled this feature "ON" in the script, Guests would be presented with a, "We encountered a problem. The reason reported was: Database error only visible to forum administrators." Admins and other registered users never saw this message, but it did appear in the logs.
The simple fix for this bug has been posted at:http://www.ubbdev.com/forums/ubbthreads.php/topics/320609.html