I experienced something similar in CPanel. Customer signs up, account is set up in CPanel, but when I try to create the Module in WHMCS I get the following error:
Module Create Failed - Service ID: 53 - Error: An unknown error occured
So I set the client's (reseller) account up in WHM and try to import it: account doesn't show up in Import list;
I downgrade the client to CPanel only, make sure the package name matches the proper settings in WHMCS and again nothing.
Only two weeks ago I imported more than hundred accounts that were transferred from another server and everything was smooth. So what's happening?