500 error on a forum when logged in only - Joomla! Forum - community, help and support

hi all, hope well.
i have forum component called discussions joomla 1.5
here is: http://www.codingfish.com/downloads/discussions/1_4_1

now, if not logged joomla can see forum , well.
as log in get:
server error
the website encountered error while retrieving http://www.mydomain.com/****/****. may down maintenance or configured incorrectly.

i have turned error reporting on , got error:
fatal error: call member function load() on non-object in
c:\inetpub\mydomain.com\wwwroot\libraries\joomla\user\user.php on line 597

the code near line 597 this:
// load jusermodel object based on user id or throw warning.
if(!$table->load($id)) {
jerror::raisewarning( 'some_error_code', 'juser::_load: unable load user id: '.$id );
return false;
}

can me? stumped!
thanks in advance.

are using social components community builder or joomsocial. plugin not compatible user extensions. however, looking @ error message assuming using standered joomla user profiles.