That did the trick. After reconfiguring the ingress controller, the session on Keycloak is terminated successfully. Maybe this is worth mentioning somewhere for the next person struggling with this? This was the post that showed the right solution for me: Fixing Nginx "upstream sent too big header" error when running an ingress controller in Kubernetes
But now I am experiencing OIDC-128 - The session is terminated (checked by looking at the page https://<my-keycloak>/auth/realms/<my-realm>/account/#/ but I am redirected to XWiki where I am still logged in. As far as I can see there is already an open PR for that?
I am using XWiki 14.10.3 deployed on Kubernetes, together with Keycloak 20.0.1 If this helps, I can provide you logs from all components involved.