3782901 - Error happens for irrelevant currency when saving PO in ME21N/ME22N - SAP S/4HANA

Below errors happen when saving or checking a PO in ME21N/ME22N:

• An exception was raised
• Evaluation of start conditions failed
• Error while evaluating custom condition '***'
• Runtime error '' during evaluation of condition '***'
• Operator '**': The value of the left operand cannot be determined
• Error in the evaluation of expression '%CL_MM_PUR_WF_OBJECT_PO<???>.CO
• Error when calling method '[X:CL:CL_MM_PUR_WF_OBJECT_PO;X:CONVERT_CURR
• Enter rate XXX / YYY rate type M for dd.mm.yyyy in the system settings
• Error in workflow simulation

Currency XXX is the PO currency but currency YYY is not used in the PO. 

• Materials Management (MM)
• SAP S/4HANA

SG105 , KBA , MM-PUR-PO-WFL , Purchase Order Workflow , Problem