Can't Pay - Wrong VAT No. : Invalid or not registered

Hello,

I can't Pay, I've got a message that says the VAT No. is wrong: Invalid or not registered, but I have choosen the right number. Can you help me?

Sometimes VAT validation service doesn't response properly (most likely because of high load) and error of validation service is considered as "not valid". Last time we found this issue, the server was restored in about 8 hours, so in this case please repeat order few hours later.
And check that VAT field contains number only without country prefix.