Lightning Preview

The Preview Web button doesn't seem to work the URL it generates is

https://<My Domain>.my.lightning.force.com/secure/frontdoor.jsp?sid=<My Session ID>..wIpkHUtyBFG4Jb2CumS9QF7zgeS.VViuFj_OPA60GHIkOKIDq2DgoQzxoSq6d45keGWh7c.v7j.yyfhkSmH3d7c&retURL=/<My Domain>/AllAccounts.app

but all I get is a 404.

The URL in my developer console is just

https://.lightning.force.com//AllAccounts.apps ..

I've read about how the new lightning experience is actually derived from their one.app, maybe this abstraction has something to do with the problem?

https://developer.salesforce.com/trailhead/lex_dev_visualforce/lex_dev_visualforce_process