Class not loaded: Ctct\ConstantContactClass in magento onepage checkout

[ReginaLisa]

I have integrated constant contact into our magento site. Its working fine in customer account creation and newsletter signup form.
But in onepage checkout billing information block which subscription checkbox, Its not working as expected.
After enter Billing information details , and click continue button, In ajax response, I got an error like

Class not loaded: Ctct\ConstantContactClass not loaded: Ctct\Services\ContactServiceClass not loaded : Ctct\Services\BaseServiceClass not loaded: Ctct\Util\RestClientClass not loaded: Ctct\Util\RestClientInterfaceClass not loaded: Ctct\Services\EmailMarketingServiceClass not loaded: Ctct\Services\ActivityServiceClass not loaded: Ctct\Services\CampaignTrackingServiceClass not loaded: Ctct\Services\ContactTrackingServiceClass not loaded: Ctct\Services\CampaignScheduleServiceClass not loaded: Ctct\Services\ListServiceClass not loaded: Ctct\Services\AccountServiceClass not loaded: Ctct\Util\ConfigClass not loaded: Ctct\Util\CurlResponseClass not loaded: Ctct\Components\Contacts\ContactClass not loaded: Ctct\Components\ComponentClass not loaded : Ctct\Components\Contacts\EmailAddressClass not loaded: Ctct\Components\ResultSetClass not loaded: Ctct \Components\Contacts\ContactListClass not loaded: Ctct\Exceptions\CtctException

Email has registered to constant contact,But Can't able to move shipping information block.
Have any idea, how to resolve this issue!
I'm sure, Listed class files were called fine!