I was having the same issue so I dug into it and found that the API permissions were not correct so you need to update the API permissions you currently gave WHMCS in CWP -> API Manager. The official guide does not mention all the permissions this module for WHMCS needs.
The official guide over at
http://wiki.centos-webpanel.com/whmcs-module-for-cwp-api states
Use the following permissions:
-add, edit, suspend, unsuspend and delete accounts
-edit packages
-change Password
I'd suggest that the CWP team changes the wording, as according to the cwp7.php included in the whmcs module, the correct permissions are as follows:
Account: del, susp, unsp, list, udp
Autologin: list
Change of password: udp
Packages: udp