I noticed that the Spaces app is available in my brand-new install, so I added a default space in preparation for adding more. However, the instant I saved the default space (name 'default', Visible To: 'All Users', Editable By: 'Administrators'), I was locked out of my install. I've salvaged things by commenting out the part of the spaces check in src/applications/base/controller/PhabricatorController.php that creates the PhabricatorSpacesNoAccessController, but it would be nice to have a real fix.

Also, I seemed to fix it when I changed to a custom policy that allowed a random user and allowed anything that didn't match one of the rules. Then, I flipped it back to 'All Users' and it continued working (without my code changes) for a day or so, for whatever reason. Super odd.