Author and Tag Feed plugins

I have just installed both the Recent Global Author Posts and Tag Feed plugins.

The Post Indexer is installed and has content.

When I try to access this URL:

feed://mydevsite.com/wp-content/tag-feed.php?tag=testtag1

... I am appropriately forwarded to:

feed://mydevsite.com/wp-content/tag-feed.php?tag=testtag1

But at that point I get a 404 — there is no content in the feed.