Error: num_rows() on a non-object

[eluser]ThaMy[/eluser]
Hi,

im using php 5.2.9 and mysql 5

in the file application/config/database.php
im using:
$db['default']['dbprefix'] = "";
$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = FALSE;
$db['default']['cache_on'] = FALSE;

if I change the value of $db['default']['db_debug'] = TRUE I get this error:

A Database Error Occurred

Unable to connect to your database server using the provided settings.

if the value of $db['default']['db_debug'] = FALSE, the page index.php open, but when I try make the login I get this error:

Call to a member function num_rows() on a non-object in /home/storage/a/dc/97/abor/public_html/sistema_abor/system/application/libraries/redux_auth.php on line 252

can anyone help me?