Sporadic "Fatal 500" error when trying to push to on gitlab omnibus instance

Summary

Developers are getting an error on push with our on premises gitlab 12.0 omnibus instance (docker hosted):

fatal: unable to access 'https://gitlab_url/team/repo.git/': The requested URL returned error: 500

So far we've only seen it happen for about a minute window, 1 time late yesterday and 1 time today (so far).

Steps to reproduce

git push from command line, TortoiseGit or Fork Git clients

What is the current bug behavior?

Sporadically fatal: unable to access error that affects all team members for maybe 30 seconds to 1 minute, then everything goes back to normal.

What is the expected correct behavior?

git push succeeds

Relevant logs and/or screenshots

(command line git push)

fatal: unable to access 'https://gitlab_url/team/repo.git/': The requested URL returned error: 500

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

root@gitlab:/# gitlab-rake gitlab:env:info
System information
System:
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:	12.0.0-ee
Revision:	3b13818e833
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	9.6.11
URL:		https://gitlab_url
HTTP Clone URL:	https://gitlab_url/some-group/some-project.git
SSH Clone URL:	ssh://git@gitlab_url:2224/some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	yes
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

root@gitlab:/# gitlab-rake gitlab:check SANITIZE=true
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: ... Server: ldapmain
(skipping server)
LDAP authentication... Success
(skipping users)
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: ...
39/4 ... yes
39/5 ... yes
74/6 ... yes
74/7 ... 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: ... 39
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

Possible fixes

Unknown