The full certificate chain order should consist of the server certificate first, followed by all intermediate certificates, with the root CA last. To fix this error, you will need to replace server’s certificate with the full chained certificate. This error indicates that an incomplete certificate chain is being presented by the server.
It is recommended to use the full certificate chain in order to prevent SSL errors when clients connect.
GitLab can be integrated with Let’s Encrypt. Service Manual SSL Let’s Encrypt Primary GitLab Instance Domain Omnibus-GitLab supports several common use cases for SSL configuration.