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

I tried setting up SSL, but I was having issues because I don't actually have DNS configured yet, I'm just using a HOSTS file locally. I tried reverting back to HTTP, but now I'm getting a 502 Bad Gateway error. It looks like Mattermost won't start.

Running gitlab-ctl status returns:

run: gitlab-workhorse: (pid 2309) 142s; run: log: (pid 737) 24887s
run: logrotate: (pid 2316) 142s; run: log: (pid 742) 24886s
down: mattermost: 0s, normally up, want up; run: log: (pid 734) 24887s
run: nginx: (pid 2336) 140s; run: log: (pid 736) 24887s
run: postgresql: (pid 2347) 139s; run: log: (pid 738) 24887s
run: redis: (pid 2349) 139s; run: log: (pid 735) 24887s
run: sidekiq: (pid 2364) 139s; run: log: (pid 739) 24887s
run: unicorn: (pid 2369) 138s; run: log: (pid 741) 24887s

As you can see, mattermost is down. If I try to run gitlab-ctl start mattermost, I get:

ok: run: mattermost: (pid 4263) 0s

But then running gitlab-ctl status mattermost, it's still down:

down: mattermost: 1s, normally up, want up; run: log: (pid 734) 24954s

I'm not sure what I changed that would be breaking this; I thought I reverted everything. The mattermost section of my gitlab.rb is here:

#####################
# GitLab Mattermost #
#####################

## Basic Settings
mattermost_external_url 'http://chat.mydomain.com'
mattermost['gitlab_enable'] = true
mattermost['gitlab_secret'] = "4**************************************************************0"
mattermost['gitlab_id'] = "c**************************************************************f"
mattermost['gitlab_scope'] = ""
mattermost['gitlab_auth_endpoint'] = "https://git.mydomain.com/oauth/authorize"
mattermost['gitlab_token_endpoint'] = "https://git.mydomain.com/oauth/token"
mattermost['gitlab_user_api_endpoint'] = "https://git.mydomain.com/api/v3/user"

## SMTP Settings
mattermost['email_feedback_name'] = "GitLab Mattermost",
mattermost['email_feedback_email'] = "git@mydomain.com",
mattermost['email_smtp_username'] = "git@mydomain.com",
mattermost['email_smtp_password'] = "************",
mattermost['email_smtp_server'] = "mydomain.com"
mattermost['email_smtp_port'] = "26"

## SSL Settings
# mattermost_nginx['redirect_http_to_https'] = true
# mattermost_nginx['ssl_certificate'] = "/etc/gitlab/ssl/mattermost-nginx.crt"
# mattermost_nginx['ssl_certificate_key'] = "/etc/gitlab/ssl/mattermost-nginx.key"
# mattermost['service_use_ssl'] = true

Could it be trying to use the same port as my HTTPS-enabled GitLab still?