AP Account Combination error upon changing the Tax code, even though the 'Allow Override' is active?

We have recently created a new Set of Books. 11.10.5.2.  Within Payables if we change the Tax Code after the Voucher has saved to somethinf other than what is creatd in the Supplier, it produces an 'Account Combination' Error.   The Tax accounting string is absolutely fine. This error only occurs when trying to update a Tax code.  If the voucher is created without changing the Tax codes the voucher is sucessful.

Does anyone know why ?

Thank you