Magento - Fatal error: spl_autoload() after adding to cart.

I have just started experiencing this problem: The site is fine until I add something to my cart at which point I start to开发者_如何学Go receive the following error at the bottom of the page:

Fatal error: spl_autoload() [<a href=’function.spl-autoload’>function.spl-autoload</a>]: Class Mage could not be loaded in /blah/blah/app/code/core/Mage/Core/functions.php on line 244

The logged in user also gets logged out and they can not log back in again.

I’ve seen a couple of forum posts that talk about this possibly being a problem with memory but my hosts (Rackspace) assure me that the memory is fine (i’ve got 16GB of RAM).

So I’m stuck.

I don't think I've changed any code to cause this. But I'm not sure.

Thanks!

I deleted the sessions in var/session/ and the problem was solved and it's working fine.

Ok. So it appears that I am storing SimpleXml Objects in the session. It seems that this is a no no...

I have removed that code and all is working ok.

I still don't know why it works fine on my development server though but at least it's working.