MIRO Message no. SG105 Enter rate / EUR rate type M for 14.04.2014 in the system settings

Dear experts,

I have this problem when I'm posting invoice for only not italian customer.

I put in MIRO invoice date, posting date, reference and PO. The system give me this error Message no. SG105 Enter rate  / EUR rate type M for 14.04.2014 in the system settings.

But if I delete the PO and without leaving the transaction I insert the same order  an other one I don't have this error anymore.

Thanks.

Nicola