500 error on specific project - imported from another repo


I have two projects that produce a 500 error when opened in a browser. According to the user both were imported from other environments (one gitorious, the other bitbucket).
Sub pages of the repos can be accessed without issue, just not the front page.

Using CE, 9.5.8.

My reading suggests that the encryption in the original environments cannot be decrypted by the keys in the gitlab secrets file. But I don’t know for sure. If this is the case then one solution is possibly to remove the ProjectImportData from the repo as its no longer necessary. Again I don’t know the steps to do this and the safety of so doing, even if it is the correct way forward.

The specific errors in gitlab-rails/production.log appear thusly:

Started GET “/jsmith/abcd” for at 2017-10-12 11:32:10 +1100
Processing by ProjectsController#show as HTML
Parameters: {“namespace_id”=>“jsmith”, “id”=>“abcd”}
Started POST “/api/v4/internal/allowed” for at 2017-10-12 11:32:10 +1100
Completed 500 Internal Server Error in 26ms (ActiveRecord: 2.2ms)

OpenSSL::Cipher::CipherError (bad decrypt):
app/models/project.rb:570:in import_url' app/models/project.rb:604:in external_import?’
app/models/project.rb:596:in import?' app/models/project.rb:616:in import_started?’
app/models/project.rb:612:in import_in_progress?' app/controllers/projects_controller.rb:97:in show’
lib/gitlab/i18n.rb:46:in with_locale' lib/gitlab/i18n.rb:52:in with_user_locale’
app/controllers/application_controller.rb:326:in set_locale' lib/gitlab/middleware/multipart.rb:93: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/request_context.rb:18:in call’
lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call’

There are several references to this problem in the gitlab developer forums but possible solutions have been about fixing gitlab code, not situations like this.

Would really appreciate any help, especially the steps required to fix.

Thank you and regards