Update: It looks like Xbox Preview owners will have their Xbox 360 games back up and running on the Xbox One soon.

Microsoft says it is working to fix a bug in the current Xbox Preview build for the Xbox One console that's preventing users from accessing their Xbox 360 backwards compatible games.

Xbox One

Microsoft Xbox team member Bill Stillwell mentioned the issues on his Twitter account:

There are now over 210 Xbox 360 games that are backwards compatible with the Xbox One, including eight games that were added to the list on Thursday.