Symptoms
Some custom developed Online Store uses the billing API methods to place orders. When using the PlanParamsGetAndValidate_API method to validate the domain contact (some User ID is passed as a value for the OwnerContactID parameter), the following error is shown:
"User ID #xxxxxxx does not belong to this account. Please provide ID of user to be used as domain contact or leave the field blank to use default user."
Cause
It is not allowed to provide as a domain contact a user ID of an account other than the order is placed for.
Resolution
Rework the custom Online Store mechanisms to call the PlanParamsGetAndValidate_API method with the correct User ID. It should belong to the same account a new order is placed for.