Well, actually, the ingressroutes where setup almost correctly.
Thanks to this thread https://community.containo.us/t/404-help-please/4121/3 , I figured out that an empty "tls" block was missing, the ssl termination is the ALB.
So adding tls: {}
did the trick, hower it was not required with a Classic LB or NLB (but I guess it should have been present.
kind: IngressRoute
apiVersion: traefik.containo.us/v1alpha1
metadata:
name: argocd-ingress
namespace: argocd
spec:
entryPoints:
- websecure
routes:
- kind: Rule
match: Host(`my.own.dns`)
services:
- name: argocd-server
port: 80
tls: {}