A user could not complete their Sprint. The message was you must have permissions to these projects (huge list) in order to complete the Sprint.
When looking at the Board filter we saw that it pulled in an Epic from another project into the Backlog. This Epic contained some Storys and relationships to other projects, but it was the only relationship to the project we were focused on.
Once we adjusted the filter to remove the Epic from the Backlog the user could complete the Sprint. We did not expect that this one Epic would be the cause of not allowing us to complete the Sprint, because it was in the Backlog. Why would that be? The Sprint and its issues are separate from the Backlog.