Accounting: booking problem

Hi guys,

we have an issue with an account booking: we have a payment to an insurance company (supplier). Saving works fine, but on submit, the following error appears (ERPNext v9.1.5):

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We have checked the default cost center in the company definition, and it is set:

The account to be paid to is defined as

The booking is from an asset account to the above expense.

Any ideas on what we are missing? How would one switch the cost center in the Payment Entry?

P.S. if we create the same booking through Supplier --> Purchase Invoice and then record a payment, it works but we are unable to modify the debit acount for the booking (it remains on the default expense account).

In this scenario, how could one change the debit account for a purchase invoice?