Problems with setting the certificate for a company intern Gitlab EE Instance run with docker

Hello everyone,
I have a problem with setting the certificate for our company intern Gitlab EE Instance.
We are running Gitlab EE 13.5.3 in a Docker Container and I want to access it via VPN.
I already got the domain validated certificates because the server won’t be accessible from the outside.

I tried a few tutorials for setting the certificates and changing the gitlab.rb file but everytime I use gitlab-ctl reconfigure I get the following errors:
letsencrypt_certificate[–gitlab-domain–] (letsencrypt::http_authorization line 5) had an error: RuntimeError: acme_certificate[staging] (/opt/gitlab/embedded/cookbooks/cache/cookbooks/letsencrypt/resources/certificate.rb line 25) had an error: RuntimeError: ruby_block[create certificate for --gitlab-domain–] (/opt/gitlab/embedded/cookbooks/cache/cookbooks/acme/resources/certificate.rb line 108) had an error: RuntimeError: [–gitlab-domain–] Validation failed, unable to request certificate

Is anybody using a similar setup and can help me?

Best regards