Fresh GitLab Installation gives error 500 on home page (Plesk, Debian 9)

I tried to install GitLab on a small personal server and configure it so that it will work on a Plesk WebAdmin subdomain. I installed GitLab normally via the Omnibus package. I changed the following settings in the gitlab.rb file:

nginx['enable'] = false
web_server['external_users'] = ['www-data', 'PLESK_USER']
web_server['group'] = 'psacln'
gitlab_workhorse['listen_network'] = "tcp"
gitlab_workhorse['listen_addr'] = "127.0.0.1:8181"

where PLESK_USER is the user linked to that plesk subdomain.

I redirected HTTP to HTTPS on the subdomain, which works, the SSL certificate also works.

In Plesk, under "additional directives for HTTPS", I pasted the text from this page in the <VirtualHost *:443> tag. https://gitlab.com/gitlab-org/gitlab-recipes/blob/master/web-server/apache/gitlab-omnibus-ssl-apache24.conf

and I replaced YOUR_SERVER_FQDN as well as the 3 SSLCertificateFile lines (with the ones I found under /var/www/vhosts/system/fqdn/conf/last_nginx.conf

Visiting my domain, I immediately get a 500 error. This is before even creating a root user. The message appears GitLab specific, having the GitLab logo and corresponding text, so I do believe the routing to GitLab itself seems to be working.

In the Production Log, I only get the following output for each site visit:

Started GET "/-/metrics" for 127.0.0.1 at 2019-05-27 11:59:44 +0000
Processing by MetricsController#index as HTML
Completed 200 OK in 2ms (Views: 0.4ms | ActiveRecord: 0.0ms | Elasticsearch: 0.0ms)

and using gitlab-ctl tail

==> /var/log/gitlab/gitlab-rails/production.log <==
Started GET "/-/metrics" for 127.0.0.1 at 2019-05-27 12:01:29 +0000
Processing by MetricsController#index as HTML
Completed 200 OK in 2ms (Views: 0.4ms | ActiveRecord: 0.0ms | Elasticsearch: 0.0ms)

==> /var/log/gitlab/gitlab-rails/production_json.log <==
{"method":"GET","path":"/-/metrics","format":"html","controller":"MetricsController","action":"index","status":200,"duration":2.64,"view":0.41,"db":0.0,"time":"2019-05-27T12:01:29.670Z","params":[],"remote_ip":null,"user_id":null,"username":null,"ua":null,"queue_duration":null,"correlation_id":"98970bd1-9618-41ea-85a2-5f59b26fd16b"}

These messages show up immediately and exactly once for every browser request I make.

sudo gitlab-rake gitlab:check --trace also doesn't reveal any further missing information. The lack of any concrete error messages leads me to believe that there is a problem with the linking between Plesk and GitLab rather than the GitLab installation itself, but I can't find this concrete problem anywhere else and I'm really out of ideas on what to check.

I'm grateful for any help.

Edit: Checking the log in /var/log/gitlab/gitlab-workhorse/current, on startup of the application (after each restart) I can see the following error messages

2019-05-27_13:11:49.39615 time="2019-05-27T13:11:49Z" level=error msg="unknown error" error="keywatcher: pubsub receive: EOF"
2019-05-27_13:11:49.39617 time="2019-05-27T13:11:49Z" level=info msg="redis: dialing" address=/var/opt/gitlab/redis/redis.socket network=unix
2019-05-27_13:11:49.39617 time="2019-05-27T13:11:49Z" level=error msg="unknown error" error="keywatcher: dial unix /var/opt/gitlab/redis/redis.socket: connect: no such file or directory"

However since I have not changed any settings past the ones I initially mentioned, I can't really find any connection to my specific configuration.