Lost all access (404) to just one project

Using GitLab CE v8.15.4, standard docker-based omnibus setup running locally.

A single project created recently (Jan 12) is now giving 404 errors and also does not allow cloning from http protocol.

Here is the production log when attempting to view the project:

Filter chain halted as :project rendered or redirected
Completed 404 Not Found in 35ms (Views: 4.2ms | ActiveRecord: 4.8ms)

All users including admin get the same errors. We have no access to the project. I have checked that a.git repo with the project name still seems to be present on the file system. GitLab is set to check integrity and reports a healthy system. The root login admin project listing still shows the project and the project size, but that’s it. I have just updated to v8.16.1, but did not change the result.

What is the next step to debug this issue? All other projects appear normal.