Got unserialize error in my frontend VM module

[hygum]

Sometimes, in my frontend module for VM cart, i get this error:
Warning: unserialize() [function.unserialize]: Function spl_autoload_call() hasn't defined the class it was called for in /home/(username)/public_html/tf1/components/com_virtuemart/helpers/cart.php on line 93

I have VM: 2.0.12f, Joomla 2.5.7, and rockettheme "rt_fracture".

What could be wrong?