First day trying new VS Community 2015 RC Version 14.0.22823.1 D14REL.
Our TFS server is still TFS 2010; I'm wondering if that's the source of the problem.
Other Team Explorer sections are working fine -- Work Items, Source Control Explorer, Pending Changes. I even did a merge and checkin successfully.
But the Builds section shows this:

I'm not even sure where to look for an error log.
VS.NET 2012, still installed on the machine, shows the Builds section correctly.
Microsoft Team Foundation Server 2010 (30319.01). I'm downloading SP1 now and will update the question with results when it's installed. - Ross Presser