Error 'Something went wrong while fetching related merge requests.' in issue details after update

Summary

I just updated my Gitlab-CE on Docker Version 11.8.0-ce.0 to 11.10.4 (62c46465). When I'm opening an issue, the error message 'Something went wrong while fetching related merge requests.' pops up. This is becaus the 'related_merge_requests?per_page=100' is fetched via HTTP and gets blocked by the browser due mixed-content policy.

Steps to reproduce

Update Gitlab on Docker from 11.8.0-ce.0 to 11.10.4 and open a Issue. We use nginx-proxy (jwilder/nginx-proxy) and letsencrypt helper (jrcs/letsencrypt-nginx-proxy-companion) to enable HTTPS on all our services. Therefore, gitlab runs on Port 80 and gets forwarded thru the proxy to port 443

What is the current bug behavior?

API is accessed via http: (http://git.company.at/api/v4/projects/13/issues/37/related_merge_requests?per_page=100) and gets blocked

What is the expected correct behavior?

Fetch data via HTTPS and do not display an error message

Results of GitLab environment info

Expand for output related to GitLab environment info

root@git:/# gitlab-rake gitlab:env:info
System information
System:
Current User:   git
Using RVM:      no
Ruby Version:   2.5.3p105
Gem Version:    2.7.6
Bundler Version:1.17.3
Rake Version:   12.3.2
Redis Version:  3.2.12
Git Version:    2.18.1
Sidekiq Version:5.2.5
Go Version:     unknown
GitLab information
Version:        11.10.4
Revision:       62c464651d2
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     9.6.11
URL:            http://git.kara-software.at
HTTP Clone URL: http://git.kara-software.at/some-group/some-project.git
SSH Clone URL:  git@git.kara-software.at:some-group/some-project.git
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers:
GitLab Shell
Version:        9.0.0
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

root@git:/# gitlab-rake gitlab:check SANITIZE=true
Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 9.0.0 ? ... OK (9.0.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
5/1 ... yes
3/2 ... yes
5/3 ... yes
3/4 ... yes
3/5 ... yes
3/7 ... yes
3/8 ... yes
5/9 ... yes
3/10 ... yes
5/13 ... yes
2/14 ... yes
5/15 ... yes
5/16 ... yes
5/17 ... yes
3/18 ... yes
5/19 ... yes
3/20 ... yes
11/21 ... yes
3/22 ... yes
5/23 ... yes
5/24 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.5.3)
Git version >= 2.18.0 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 8
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished