There is no "Login to Plesk Control Panel" button for customers in WHMCS client area.
One of reseller's credentials are used to integrate with Plesk for an affected server (
Setup -> Products/Services -> Servers -> Choose an affected server -> Edit).
WHMCS is trying to create session for a customer or a reseller using credentials from another reseller.
Since MU #43 the above described behavior is considered as expected. The following user roles are set for WHMCS:
Admin is able to create sessions for admin, reseller, customer. Reseller is able to create session for himself. Customer is able to create session for himself. Reseller is not able to create session for admin, any customer or another reseller Customer is not able to create session for admin, any reseller or another customer Login is possible with created session for reseller, customer.
Use Plesk "admin" credentials to integrate Plesk and WHMCS.
In a case the "Login to Plesk Control Panel" button is missed for the reseller which credentials are used to integrate Plesk and WHMCS please check the following KB article: