Reseller's accounts at Openprovider use default handles as by design, this way default handles are mandatory and should exist in every account permanently.
However, often one may face a situation when specific default handle deletion is needed. On attempt to delete this handle the following error occurs: "Cannot delete a customer that assigned as default handle(s)."
You may check if the handle is used as default by clicking "Show use" button
at handle details https://rcp.openprovider.eu/web/action/index#/customer/overview or navigating to https://rcp.openprovider.eu/account/config-billing-handle.php.
In order to have the exact default handle deleted first you need to re-assign the default handle to any handle registered within your account and after that you will be able to delete the handle that was used as default.
Please use this page to re-assign the default handle https://rcp.openprovider.eu/account/config-billing-handle.php
After that you will be able to delete that specific handle.