Error loading burndown chart data for any milestone

Summary

We getting the following error for any milestones (already existing & new)

Error loading burndown chart data

(12.1.3-ee starter)

Steps to reproduce

Just open any milestone!

What is the current bug behavior?

Error Error loading burndown chart data is shown but no burndown chart

What is the expected correct behavior?

No error message should be shown and the burndown chart should be displayed

Relevant logs and/or screenshots

Screenshot_from_2019-08-02_22-57-53

Results of GitLab environment info

Expand for output related to GitLab environment info 
System information
System:		Ubuntu 18.04
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.6.3p62
Gem Version:	2.7.9
Bundler Version:1.17.3
Rake Version:	12.3.2
Redis Version:	3.2.12
Git Version:	2.21.0
Sidekiq Version:5.2.7
Go Version:	unknown

GitLab information
Version:	12.1.3-ee
Revision:	a05c811e614
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	10.7
URL:		[concealed]
HTTP Clone URL:	[concealed]
SSH Clone URL:	[concealed]
Elasticsearch:	no
Geo:		no
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers:


GitLab Shell
Version:	9.3.0
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 
Checking GitLab subtasks ...

Checking GitLab Shell ...


GitLab Shell: ... GitLab Shell version >= 9.3.0 ? ... OK (9.3.0)
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: ...
3/1 ... yes
3/2 ... yes
3/3 ... yes
3/4 ... yes
3/5 ... yes
3/6 ... yes
3/7 ... yes
3/8 ... yes
3/9 ... yes
3/10 ... yes
3/11 ... yes
3/12 ... yes
1/13 ... yes
3/14 ... yes
3/15 ... yes
3/16 ... yes
3/18 ... yes
3/19 ... yes
3/20 ... yes
1/21 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.3)
Git version >= 2.21.0 ? ... yes (2.21.0)
Git user has default SSH configuration? ... yes
Active users: ... 4
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)


Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished
Edited by Jan Schwien