Editing old snippets fail after backup and restore

I’ve moved a GitLab CE 13.4.1 omnibus instance from Ubuntu 16.04 to 20.04 via backup and restore.

Attempting to edit a snippet that was present on the old system returns an error when saving:

Can't update snippet: Repository Error updating the snippet

I see this in /var/log/gitlab/gitlab-rails/production.log when the error occurs:

Started POST "/api/graphql" for 198.187.188.6 at 2020-09-27 11:10:25 -0700
Processing by GraphqlController#execute as */*
  Parameters: {"_json"=>[{"operationName"=>"UpdateSnippet", "variables"=>"[FILTERED]", "query"=>"mutation UpdateSnippet($input: UpdateSnippetInput!) {\n  updateSnippet(input: $input) {\n    errors\n    snippet {\n      webUrl\n      __typename\n    }\n    __typename\n  }\n}\n"}], "graphql"=>{"_json"=>[{"operationName"=>"UpdateSnippet", "variables"=>"[FILTERED]", "query"=>"mutation UpdateSnippet($input: UpdateSnippetInput!) {\n  updateSnippet(input: $input) {\n    errors\n    snippet {\n      webUrl\n      __typename\n    }\n    __typename\n  }\n}\n"}]}}
{:message=>"Snippet git error. Reason: A file with this name doesn't exist", :snippet=>1}
Completed 200 OK in 823ms (Views: 0.2ms | ActiveRecord: 325.3ms | Elasticsearch: 0.0ms | Allocations: 19281)

Output from sudo gitlab-rake gitlab:check:

Checking GitLab subtasks ...

Checking GitLab Shell ...

GitLab Shell: ... GitLab Shell version >= 13.7.0 ? ... OK (13.7.0)
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: ... Checking Reply by email ...

IMAP server credentials are correct? ... Checking gitlab@xxx.com
yes
Init.d configured correctly? ... skipped
MailRoom running? ... skipped

Checking Reply by email ... Finished


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? ... 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: ... 
Airporter / reservations ... yes
Airporter / reslite ... yes
GDAS / gdas ... yes
Aoife Finch / create-domain-uml ... yes
Aoife Finch / pomgrp ... yes
Aoife Finch / pomgrp-cas ... yes
Aoife Finch / tsw-adventure-editor ... yes
Aoife Finch / ccm ... yes
Aoife Finch / cgi-bin ... yes
Aoife Finch / dixie ... yes
GDAS / gdasod ... yes
Aoife Finch / redmine ... yes
GDAS / teigha-loader ... yes
GDAS / teigha-artifacts ... yes
GDAS / gdas-agent ... yes
Aoife Finch / patentleverage ... yes
Aoife Finch / pearson ... yes
Aoife Finch / robolson ... yes
Aoife Finch / router-tutorial ... yes
packages / authentication ... yes
Airporter / ccm ... yes
Aoife Finch / sieve ... yes
GDAS / gdas-web ... yes
Aoife Finch / test-project ... yes
Airporter / admin ... yes
Airporter / cgi-bin ... yes
Airporter / htdocs ... yes
Airporter / orders ... yes
Airporter / plone ... yes
Airporter / pyplugin ... yes
alphasoft / control-repo ... yes
alphasoft / ssh-alpine ... yes
alphasoft / infrastructure ... yes
Airporter / network-info ... yes
Aoife Finch / dazzler ... yes
Airporter / legacy ... yes
GDAS / putenv ... yes
alphasoft / time-report ... yes
Airporter / apache-config ... yes
alphasoft / asterisk-config ... yes
alphasoft / logwatch ... yes
Aoife Finch / test4 ... yes
GDAS / teigha-activation ... yes
Aoife Finch / cardreader ... yes
alphasoft / rainloop ... yes
GDAS / teigha-interface ... yes
GDAS / infrastructure ... yes
packages / gradle-k8s-plugins ... yes
alphasoft / nyssa-admin ... yes
alphasoft / external-storage ... yes
alphasoft / external-dns ... yes
alphasoft / k3s-admin ... yes
Redis version >= 4.0.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.6)
Git version >= 2.24.0 ? ... yes (2.28.0)
Git user has default SSH configuration? ... yes
Active users: ... 15
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

Output from sudo gitlab-rake gitlab:env:info:

System information
System:		Ubuntu 20.04
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:	2.28.0
Sidekiq Version:5.2.9
Go Version:	unknown

GitLab information
Version:	13.4.1
Revision:	c90be62bdef
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	11.9
URL:		https://xxx.com
HTTP Clone URL:	https://xxx.com/some-group/some-project.git
SSH Clone URL:	git@xxx.com:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	13.7.0
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