Federated cloud shares

Server configuration
Operating system: Debian Jessie 8
Web server: Apache
Database: MySQL
PHP version: PHP 7.0.17-1~dotdeb+8.1
ownCloud version (see ownCloud admin page): 9.1.4
Updated from an older ownCloud or fresh install: Fresh install
Special configuration (external storage, external authentication, reverse proxy, server-side-encryption): No special configuration

I have two Owncloud instances. I would like to establish a federated shares between then. SSL is enabled on both instances using Let's encrypt certificates. I've added both servers in the federation section in the admin page. The problem is that both servers are in the yellow state and it never turned to green. Where should I look to debug the problem?