Access to mattermost when not using the integrated NGINX server


I’m using the ominbus-ce version on an Ubuntu server. I deactivated the internal NGINX server (nginx['enable'] = false in /etc/gitlab/gitlab.rb) to use the one I installed, as I need some other virtual hosts.
I created a virtualhost in NGINX for the Gitlab part, which works perfectly, but I couldn’t find any way to make a similar one for the Mattermost part. I copied the virtualhost I had for gitlab to another one with the domain named as mattermost.XXXX (which matches the definition in the conf file gitlab.rb :

mattermost_external_url https://mattermost.XXXX

But when I access https://mattermost.XXXX, I still get redirected to the gitlab part.

How can I debug this?