Mattermost config error: Failed to update assets subpath from config

Summary

I have Mattermost installed as part of the gitlab omnibus. Mattermost logs an error relating to configuration of paths.

/var/log/gitlab/mattermost/mattermost.log:{"level":"error","ts":1591703023.1573215,"caller":"web/static.go:25","msg":"Failed to update assets subpath from config","error":"failed to update root.html with subpath /: open /opt/gitlab/embedded/service/mattermost/client/root.html: permission denied","errorVerbose":"open /opt/gitlab/embedded/service/mattermost/client/root.html: permission denied\nfailed to update root.html with subpath /\ngithub.com/mattermost/mattermost-server/v5/utils.UpdateAssetsSubpath\n\tgithub.com/mattermost/mattermost-server/v5@/utils/subpath.go:117\ngithub.com/mattermost/mattermost-server/v5/utils.UpdateAssetsSubpathFromConfig\n\tgithub.com/mattermost/mattermost-server/v5@/utils/subpath.go:162\ngithub.com/mattermost/mattermost-server/v5/web.(*Web).InitStatic\n\tgithub.com/mattermost/mattermost-server/v5@/web/static.go:24\ngithub.com/mattermost/mattermost-server/v5/web.New\n\tgithub.com/mattermost/mattermost-server/v5@/web/web.go:39\ngithub.com/mattermost/mattermost-server/v5/cmd/mattermost/commands.runServer\n\tgithub.com/mattermost/mattermost-server/v5@/cmd/mattermost/commands/server.go:77\ngithub.com/mattermost/mattermost-server/v5/cmd/mattermost/commands.serverCmdF\n\tgithub.com/mattermost/mattermost-server/v5@/cmd/mattermost/commands/server.go:53\ngithub.com/spf13/cobra.(*Command).execute\n\tgithub.com/spf13/cobra@v0.0.5/command.go:826\ngithub.com/spf13/cobra.(*Command).ExecuteC\n\tgithub.com/spf13/cobra@v0.0.5/command.go:914\ngithub.com/spf13/cobra.(*Command).Execute\n\tgithub.com/spf13/cobra@v0.0.5/command.go:864\ngithub.com/mattermost/mattermost-server/v5/cmd/mattermost/commands.Run\n\tgithub.com/mattermost/mattermost-server/v5@/cmd/mattermost/commands/root.go:15\nmain.main\n\tgithub.com/mattermost/mattermost-server/v5@/cmd/mattermost/main.go:31\nruntime.main\n\truntime/proc.go:203\nruntime.goexit\n\truntime/asm_amd64.s:1357"}

The permissions and ownership of the file it mentions show that nobody except root can write to this file:

-rw-r--r-- 1 root root 3657 Jun  4 16:45 /opt/gitlab/embedded/service/mattermost/client/root.html

From the logs, it looks like this error happens repeatedly, or at least on every startup of Mattermost.

I don't know what the significance of this is - I've not noticed any particular effects - but it just sounds like it should not be happening.

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Debian 8.11
Current User:   git
Using RVM:      no
Ruby Version:   2.6.6p146
Gem Version:    2.7.10
Bundler Version:1.17.3
Rake Version:   12.3.3
Redis Version:  5.0.9
Git Version:    2.26.2
Sidekiq Version:5.2.7
Go Version:     unknown

GitLab information
Version:        13.0.5
Revision:       9097f912ba5
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     11.7
URL:            https://git.example.com
HTTP Clone URL: https://git.example.com/some-group/some-project.git
SSH Clone URL:  git@ git.example.com:some-group/some-project.git
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers:

GitLab Shell
Version:        13.2.0
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 >= 13.2.0 ? ... OK (13.2.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: 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: ...
2/1 ... yes
2/2 ... yes
14/4 ... yes
14/5 ... yes
2/6 ... yes
17/8 ... yes
14/9 ... yes
2/10 ... yes
14/11 ... yes
14/12 ... yes
14/13 ... yes
14/14 ... yes
Redis version >= 4.0.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.6)
Git version >= 2.22.0 ? ... yes (2.26.2)
Git user has default SSH configuration? ... yes
Active users: ... 27
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished