What settings to use to get iDeal to work with Marktetpress

The iDeal settings in MarketPress call for

Bank

Merchant ID

Secret iDeal key

On the Merchant ID, I have both a number and a name. Which one to use? And the Secret iDeal key is that the same as the Certificate fingerprint?

The error message I receive, both with the number and name for the Merchant ID is:

A technical error occurred.

Ongeldig acceptant ID of acceptant niet geregistreerdvoor gebruik van deze betaalmethode

My iDeal account is active and I have Marketpress 2.7 installed