Error 500 on my vps server, but when i refresh its just running normally

i dont know why i get error like this, so when my server is on production my server usually get an error, but when i refresh its just going well, but i get this error in many times, i've tried everything but its still like that, whats the problem, and when i try to check it the error is like this

A PHP Error was encountered

Severity: Warning
Message: Attempt to read property "db" on null
Filename: Session/Session.php
Line Number: 812
Backtrace:
File: /var/www/html/zonaapoteker/application/controllers/Home.php
Line: 18
Function: set_userdata
File: /var/www/html/zonaapoteker/index.php
Line: 307
Function: require_once
An uncaught Exception was encountered
Type: Error
Message: Call to a member function empty_table() on null
Filename: /var/www/html/zonaapoteker/system/libraries/Session/Session.php
Line Number: 812
Backtrace:
File: /var/www/html/zonaapoteker/application/controllers/Home.php
Line: 18
Function: set_userdata
File: /var/www/html/zonaapoteker/index.php
Line: 307
Function: require_once

1) What type of Session's are you using?

Database, File or etc.

2) How and where did you initialize your Session's?

What did you Try? What did you Get? What did you Expect?

Joined CodeIgniter Community 2009.  ( Skype: insitfx )