ActionView::Template::Error (bad decrypt) / verified_at = import_data.ssh_known_hosts_verified_at

Summary

With gitlab-10.5.4 we noticed that we are no longer able to open /settings/repository on a specific project. Instead it errors with a 500. It works fine on all other projects.

Steps to reproduce

Open https://gitlab.foo.bar/some-group/some-project/settings/repository

What is the current bug behavior?

500 - Whoops, something went wrong on our end.

What is the expected correct behavior?

page responds with 200 and shows content

Relevant logs and/or screenshots

Completed 500 Internal Server Error in 66ms (ActiveRecord: 16.6ms | Elasticsearch: 0.0ms)
ActionView::Template::Error (bad decrypt):

1: - import_data = f.object
2: - verified_by = import_data.ssh_known_hosts_verified_by
3: - verified_at = import_data.ssh_known_hosts_verified_at
4:
5: .form-group.js-ssh-host-keys-section{ class: ('hidden' unless import_data.ssh_import?) }

app/views/projects/settings/repository/show.html.haml:10:in `_app_views_projects_settings_repository_show_html_haml__164674049815214345_70125114160120'
lib/gitlab/i18n.rb:47:in `with_locale'
lib/gitlab/i18n.rb:53:in `with_user_locale'
app/controllers/application_controller.rb:333:in `set_locale'
lib/gitlab/middleware/multipart.rb:95:in `call'
lib/gitlab/request_profiler/middleware.rb:14:in `call'
lib/gitlab/middleware/go.rb:17:in `call'
lib/gitlab/etag_caching/middleware.rb:11:in `call'
lib/gitlab/middleware/read_only.rb:31:in `call'
lib/gitlab/request_context.rb:18:in `call'
lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

$ gitlab-rake gitlab:env:info

System information
System:
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.3.6p384
Gem Version:    2.6.13
Bundler Version:1.13.7
Rake Version:   12.3.0
Redis Version:  3.2.11
Git Version:    2.14.3
Sidekiq Version:5.0.5
Go Version:     unknown

GitLab information
Version:        10.5.4-ee
Revision:       18e79be
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
DB Version:     9.6.6
URL:            https://gitlab.foo
HTTP Clone URL: https://gitlab.foo/some-group/some-project.git
SSH Clone URL:  git@git.foo:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: no

GitLab Shell
Version:        6.0.3
Repository storage paths:
- default:      /gitlab-data/foo/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks
Git:            /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

gitlab-rake gitlab:check SANITIZE=true reports everything green, no errors

Possible fixes

It looks similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/13590 but I can't figure out which database table is the source of this bug.