> rather an Access Control Server (ACS) that your bank uses. Ideally, this page should reside under the bank's domain name
To me it looks like it's another company than "La Banque Postale": "Worldline is a French multinational payment and transactional services company founded in 1974."
https://fr.wikipedia.org/wiki/Worldline
Whois of wlp-acs.com:
Organization:WORLDLINE FRANCE
Street:80 Quai Voltaire Immeuble River Ouest
City:Bezons
State:FR
Postal Code:95870