git clone via ssh not working

Summary

Used versions

gitlab-ce 11.5.0
centos 7 -> git --version -> 1.8.3.1
mac os mojave -> git --version -> 2.17.2 (Apple Git-113)

I've setup gitlab-ce on centos (git already installed on centos machine), created a new project and added my ssh public key inside the user settings profile. When going to settings -> members, my user is tagged as Maintainer. I'm trying to clone the repo with git clone git@my.domain.net:username/my-project.git. But I get:

Cloning into 'my-project'...
fatal: 'username/my-project.git' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

When executing: ssh git@my.domain.net, I immediately see something like

Last login: Tue Nov 27 10:45:07 2018 from IPADDRESS
-sh-4.2$

So this seems to work.

When I switch the dropdown to http inside gitlab and enter the suggested git clone command: git clone http://... and enter the username and password I've cloned the project successfully. I was also able to create the README.md and push it back.

Environement details: Gitlab is only accessible inside the own vpn/intranet (not a public available domain). I've taken the Omnibus installation but used an external/unbound nginx. Gitlab authentication via LDAP. Don't know if the already installed git somehow interferes into gitlab-ce.

Steps to reproduce

Create new project
Select ssh in the dropdown and clone it -> git clone git@my.domain.net:username/my-project.git You see the above mentioned error.
Select http in the dropdwon and and clone it -> git clone http://... You see it is working without any errors.

What is the current bug behavior?

git clone git@my.domain.net:username/my-project.git via ssh NOT possible, although a) ssh git@my.domain.net works and b) git clone http://... works

What is the expected correct behavior?

git clone git@my.domain.net:username/my-project.git via ssh IS possible

Results of GitLab environment info

Expand for output related to GitLab environment info

sudo gitlab-rake gitlab:env:info

System information
System:		
Current User:	git
Using RVM:	no
Ruby Version:	2.4.5p335
Gem Version:	2.7.6
Bundler Version:1.16.6
Rake Version:	12.3.1
Redis Version:	3.2.12
Git Version:	2.18.1
Sidekiq Version:5.2.1
Go Version:	unknown

GitLab information
Version:	11.5.0
Revision:	b7b1e8e
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		http://my.domain.net
HTTP Clone URL:	http://my.domain.net/some-group/some-project.git
SSH Clone URL:	git@my.domain.net:some-group/some-project.git
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	8.4.1
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

Expand for output related to the GitLab application check

sudo gitlab-rake gitlab:check SANITIZE=true

Checking GitLab Shell ...

GitLab Shell version >= 8.4.1 ? ... OK (8.4.1)
hooks directories in repos are links: ... 
2/1 ... ok
2/2 ... ok
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 ...

default ... OK

Checking Gitaly ... Finished

Checking Sidekiq ...

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

Checking Sidekiq ... Finished

Reply by email is disabled in config/gitlab.yml
Checking LDAP ...

Server: ldapmain
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
	DN: cn=user name,cn=internal,ou=users,dc=domain,dc=com	 mail: user.name@domain.com

Checking LDAP ... Finished

Checking GitLab ...

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? ... skipped (no tmp uploads folder yet)
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: ... 
2/1 ... yes
2/2 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.4.5)
Git version >= 2.9.5 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 2

Checking GitLab ... Finished