Changing from HTTP to HTTPS results in ERR_CONNECTION_REFUSED for both Mattermost & GitLab

Changing the mattermost_external_url in my gitlab.rb to https instead of http (and then running sudo gitlab-ctl reconfigure) results in an "ERR_CONNECTION_REFUSED" error in Chrome, for both Mattermost and GitLab. No idea why this would be happening, seems odd that it's afffecting my GitLab instance as well.

I'm using Ubuntu 15.04, and I have Let's Encrypt set up for my GitLab instance. I haven't generated any SSL certificates for my Mattermost instance yet.

Here's a screenshot of the error.

Any idea why this would be happening?

EDIT: I just checked my System Console, and I think this is the error it's producing when it's set to HTTPS: Failed to get security update information from Mattermost.

EDIT 2: I just had another thought about this: Could it be causing issues because Mattermost rules on a separate port, and trying to set it to HTTPS is trying to use the same port as GitLab?