PlantUML integration not working - blocks in Markdown stay as-is

Summary

I have a running PlantUML server, and added the config option in the admin area. However tests in either issues or wiki page do not show PNGs replacing the plantuml code blocks, as explained in the documentation.

Steps to reproduce

Install PlantUML server
Add the configuration in the admin area with the URL to the server
Add the 'Alice & Bob' example from the documentation page into a new Wiki page from an existing project

What is the current bug behavior?

The example is rendered as generic code block output

What is the expected correct behavior?

The example should be rendered as a PNG

Relevant logs and/or screenshots

That's my problem - I cannot see any connection attempt to the PlantUML servers in the logs.

Output of checks

Results of GitLab environment info

# gitlab-rake gitlab:env:info

System information
System:         Ubuntu 14.04
Current User:   gitlab
Using RVM:      no
Ruby Version:   2.3.3p222
Gem Version:    2.6.6
Bundler Version:1.13.7
Rake Version:   10.5.0
Redis Version:  3.2.5
Git Version:    2.10.2
Sidekiq Version:4.2.7

GitLab information
Version:        9.0.5
Revision:       a6b9899
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            https://<machine fqdn>
HTTP Clone URL: https://<machine fqdn>/some-group/some-project.git
SSH Clone URL:  gitlab@<machine fqdn>:some-group/some-project.git
Using LDAP:     yes
Using Omniauth: no

GitLab Shell
Version:        5.0.0
Repository storage paths:
- default:      /var/opt/gitlab/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:            /opt/gitlab/embedded/bin/git

Results of GitLab application Check

# gitlab-rake gitlab:check SANITIZE=true
Checking GitLab Shell ...

GitLab Shell version >= 5.0.0 ? ... OK (5.0.0)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by gitlab:gitlab?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
3/1 ... ok
3/3 ... ok
5/5 ... ok
5/6 ... ok
5/7 ... ok
5/8 ... ok
5/9 ... ok
5/13 ... ok
5/14 ... ok
4/21 ... ok
4/36 ... ok
4/37 ... ok
4/38 ... ok
5/40 ... ok
5/41 ... ok
2/45 ... ok
3/48 ... ok
19/50 ... repository is empty
19/51 ... repository is empty
19/52 ... repository is empty
2/53 ... ok
2/54 ... ok
13/56 ... ok
2/57 ... ok
2/58 ... ok
5/60 ... ok
2/61 ... ok
2/63 ... ok
18/64 ... repository is empty
13/65 ... ok
2/66 ... ok
13/67 ... ok
13/68 ... ok
2/70 ... ok
13/71 ... ok
10/72 ... ok
2/73 ... ok
2/74 ... ok
13/75 ... ok
13/76 ... ok
13/78 ... ok
13/79 ... ok
17/80 ... ok
17/81 ... ok
17/82 ... ok
17/83 ... ok
8/84 ... ok
17/87 ... ok
2/88 ... ok
5/89 ... ok
4/90 ... repository is empty
3/91 ... ok
5/92 ... ok
5/93 ... ok
62/94 ... ok
62/95 ... repository is empty
5/96 ... ok
5/99 ... ok
62/100 ... ok
5/101 ... ok
62/103 ... ok
13/104 ... ok
71/105 ... ok
2/106 ... ok
5/107 ... ok
5/115 ... ok
72/116 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
Send ping to redis server: OK
gitlab-shell self-check successful

Checking GitLab Shell ... Finished

Checking Sidekiq ...

Running? ... yes
Number of Sidekiq processes ... 1

Checking Sidekiq ... Finished

Checking Reply by email ...

Reply by email is disabled in config/gitlab.yml

Checking Reply by email ... Finished

Checking LDAP ...

Server: ldapmain
LDAP authentication... Anonymous. No `bind_dn` or `password` configured
LDAP users with access to your GitLab server (only showing the first 100 results)
<sanitized>

Checking LDAP ... Finished

Checking GitLab ...

Git configured with autocrlf=input? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory setup correctly? ... 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: ...
3/1 ... yes
3/3 ... yes
5/5 ... yes
5/6 ... yes
5/7 ... yes
5/8 ... yes
5/9 ... yes
5/13 ... yes
5/14 ... yes
4/21 ... yes
4/36 ... yes
4/37 ... yes
4/38 ... yes
5/40 ... yes
5/41 ... yes
2/45 ... yes
3/48 ... yes
19/50 ... yes
19/51 ... yes
19/52 ... yes
2/53 ... yes
2/54 ... yes
13/56 ... yes
2/57 ... yes
2/58 ... yes
5/60 ... yes
2/61 ... yes
2/63 ... yes
18/64 ... yes
13/65 ... yes
2/66 ... yes
13/67 ... yes
13/68 ... yes
2/70 ... yes
13/71 ... yes
10/72 ... yes
2/73 ... yes
2/74 ... yes
13/75 ... yes
13/76 ... yes
13/78 ... yes
13/79 ... yes
17/80 ... yes
17/81 ... yes
17/82 ... yes
17/83 ... yes
8/84 ... yes
17/87 ... yes
2/88 ... yes
5/89 ... yes
4/90 ... yes
3/91 ... yes
5/92 ... yes
5/93 ... yes
62/94 ... yes
62/95 ... yes
5/96 ... yes
5/99 ... yes
62/100 ... yes
5/101 ... yes
62/103 ... yes
13/104 ... yes
71/105 ... yes
2/106 ... yes
5/107 ... yes
5/115 ... yes
72/116 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.3.3)
Your git bin path is "/opt/gitlab/embedded/bin/git"
Git version >= 2.7.3 ? ... yes (2.10.2)
Active users: 84

Checking GitLab ... Finished