It happen when you or your customers have updated their email with their profile but with their client user still old email. To fix it follow below steps:
- Login as your WHMCS admin
- Go to clients list or search for specific customer
- Go to "Users" tab with customer profile inside admin
- Click on Manage User button for account type "Owner"
- Update Email Address
- Click on Save Changes

After updating the email for owner user please reset password for client from summary tab and try to login. It will work.