So this is apparently The Schwarz Group, a parent company of Lidl and Kaufland, and it seems StackIT is their IT division.
Just for fun, just tried two times their online process to open an account, from two difference devices. Send email, get challenge code...bla bla..to be immediately greeted two times at validation time by:
"No authentication process found. Please try again or contact support for assistance."
This is exactly the cluster fuck you would expect from IT run by a German company ( Hetzner being the exception...). Good luck Dutch Central Bank...