I have got an answer from Rabo OmniKassa:
Unfortunately due to a technical issue our test enviroment is not working right now.
Under normal circumstances, you can integrate the webshop with the Rabo OmniKassa test environment, by using the following data for the test webshop:
Simulation URL connector: https://payment-webinit.simu.omnikassa.rabobank.nl/paymentServlet
You can use these variables:
We hope to resolve this as soon as possible.
Sorry for the inconvenience.