User/groups disappear for project based security?

[RiRL]

I am seeing an issue that when I go to the configuration page for a project with Project-based security enabled: after an initial load of the page, the user/group names disappear, and access is lost as a result.

I cannot clear these settings or revert the configuration change or make any other changes. 

This happens for only 1 of many projects using this security setting in this environment. Others are fine.

Has anyone seen this issue? Is there a way to reset the security settings on this configuration only?

I suppose as a last restore we will need to edit the corresponding config.xml directly on the server and reload Jenkins?