This is likely caused by D14825 and may even be intentional. I can't reproduce it on this install though (https://secure.phabricator.com/tag/bug_report/ correctly takes me to the project detail page). On my private install, however, I've noticed that /tag/X (where X is a project that does not have a workboard initialized) redirects (with a 302) to /tag//, which is a 404.

I am at HEAD (updated a few hours ago) and all storage migrations have been applied.