400 bad request in mattermost

From here I tried this in a new test installation. In a new Ubuntu 16.04 virtual machine, install GitLab as in the official guide:

sudo apt-get install curl openssh-server ca-certificates postfix
curl -sS https://packages.gitlab.com/install/repositories/gitlab/gitlab-ce/script.deb.sh | sudo bash
sudo apt-get install gitlab-ce

At this point, GitLab works fine at http://127.0.0.1.

Edit your /etc/gitlab/gitlab.rb:

sudo nano /etc/gitlab/gitlab.rb

If the machine is called "Ubuntu16", the external_url is:

external_url 'http://Ubuntu16'

Change it for this:

external_url 'http://Ubuntu16:802'

Save (Ctrl+X, Y, Enter) and reconfigure:

sudo gitlab-ctl reconfigure

At this point, GitLab works fine at http://127.0.0.1.:802.

Now edit your /etc/gitlab/gitlab.rb:

sudo nano /etc/gitlab/gitlab.rb

And change your mattermost_external_url for this:

mattermost_external_url 'http://Ubuntu16:8065'

Save (Ctrl+X, Y, Enter) and reconfigure:

sudo gitlab-ctl reconfigure

Now, if you go to http://127.0.0.1.:8065 you get:

400 Bad Request
Request Header Or Cookie Too Large
nginx