Exception: Serialization of 'SimpleXMLElement' is not allowed in serialize()

My drupal version is 7.22. I am using entity registration(7.x-1.1). I didn't use field collection at the begining but now i need to add field collection. After adding field collection(7.x-1.0-beta7) to my entity registration form, my site crash and display this error "Exception: Serialization of 'SimpleXMLElement' is not allowed in serialize() (line 450 of /var/www/drupal/includes/cache.inc).".
I used unlimited in "Number of values " option. But when i used static no of items in "Number of values " drop down this issue disapears and registration form work good.
Please help me out here. i need to use unlimited field for that field collection.