Federation and SSL self signed error

Hello all,
I have two new Nextcloud 9.0.52 server both with ssl, and only ssl, enabled.
I have federated the two servers but when i try to share a directory from one server to the another i cannot see the directory shared and I have the following error in the logs:

Exception while scanning storage “shared::91d082021a1c5dfe063098bec2167b88”: Sabre\HTTP\ClientException: SSL certificate problem: self signed certificate in certificate chain

I have an internal CA and I’ve copied my root (and intermediate) CA in the $NEXTCLOUD_PATH/resources/config/ca-bundle.crt
without success. Disabling ssl everything works fine.
What’s wrong with what I am doing?

Kind regards

Summoning our security overload @LukasReschke