Gitlab-CE 403 Forbidden when accessed via IPv6 behind Apache2 Reverse Proxy


I’ve encountered a problem trying to access Gitlab-CE (Omnibus) via IPv6 behind an Apache2 Reverse Proxy.
If a client tries to access the page via IPv6 it receives a 403 forbidden page, via IPv4 everything works as expected.
As a quick workaround I disabled IPv6 on my server.

I can’t say if it’s an issue with my Apache config or with Gitlab.
My Apache config is based on a recipe to put Gitlab behind an Apache2 Reverse Proxy with SSL (for Omnibus package)

Nginx is disabled, workhorse is operating on port 8181
Apache config:

DocumentRoot /somewhere
ErrorLog /var/log/gitlab.example.com_error_log
CustomLog /var/log/gitlab.example.com_access_log combined
unimportant stuff
SSLEngine on
SSLCertificateFile /ssl.cert
SSLCertificateKeyFile /ssl.key
SSLCACertificateFile /

#strong encryption ciphers only
#see ciphers(1)
SSLProtocol all -SSLv2
SSLHonorCipherOrder on
Header add Strict-Transport-Security: "max-age=15768000;includeSubdomains"
SSLCompression Off

# proxy settings
ProxyPreserveHost On
SSLProxyEngine On
SSLProxyCheckPeerCN On
SSLProxyCheckPeerExpire On
ProxyRequests Off
RemoteIPHeader X-Forwarded-For

# Ensure that encoded slashes are not decoded but left in their encoded state.
AllowEncodedSlashes NoDecode

<Location />
  # New authorization commands for apache 2.4 and up
  Require all granted

  #Allow forwarding to gitlab-workhorse

# Apache equivalent of nginx try files
RewriteEngine on

#Don't escape encoded characters in api requests
RewriteCond %{REQUEST_URI} ^/api/v3/.*
RewriteRule .*{REQUEST_URI} [P,QSA,NE]

#Forward all requests to gitlab-workhorse except existing files like error documents
RewriteCond %{REQUEST_URI} ^/uploads/.*
RewriteRule .*{REQUEST_URI} [P,QSA]

RequestHeader set X_FORWARDED_PROTO 'https'
RequestHeader set X-Forwarded-Ssl on

#Set up apache error documents, if back end goes down (i.e. 503 error) then a maintenance/deploy page is thrown up.
ErrorDocument 404 /404.html
ErrorDocument 422 /422.html
ErrorDocument 500 /500.html
ErrorDocument 502 /502.html
ErrorDocument 503 /503.html